Impact of COVID-19 on live birth and perinatal outcomes following subsequent frozen-thawed embryo transfer

Purpose To evaluate whether prior SARS-CoV-2 infection adversely affects live birth and perinatal outcomes following subsequent frozen-thawed embryo transfer (FET) and to further investigate whether the time interval between infection and embryo transfer influences pregnancy outcomes.

In this retrospective cohort study, women who tested positive for SARS-CoV-2 before FET were designated as the infected group (n = 1081), while asymptomatic and test-negative women were included as controls (n = 865). The infected group was further subdivided into four subgroups according to the time interval from infection to FET: ≤ 30, 31–60, 61–90, and ≥ 91 days.

Overall, positive HCG rate was significantly lower in the infected group compared to the non-infected group (62.9% vs. 67.5%, adjusted OR = 0.80, 95% CI = 0.66–0.97, P = 0.021), while no significant differences were observed in live birth rates (45.9% vs. 49.7%, adjusted OR = 0.84, 95% CI = 0.70–1.01, P = 0.062). There were no significant differences in clinical pregnancy, pregnancy loss, or any of the measured obstetric and neonatal outcomes between groups. However, women in the ≤ 30 days’ subgroup were associated with a significant reduction in positive HCG (adjusted OR = 0.52, 95% CI = 0.33–0.84, P = 0.007), clinical pregnancy (adjusted OR = 0.60, 95% CI = 0.38–0.95, P = 0.029), and live birth rates (adjusted OR = 0.51, 95% CI = 0.31–0.83, P = 0.006) compared to those in the non-infected group, while no such association was observed with the other subgroups.

In those undergoing FET, overall, prior SARS-CoV-2 infection does not adversely affect live birth and perinatal outcomes. However, considering the decreased likelihood of live birth among women with shorter intervals, delaying the subsequent FET for at least 30 days after infection could be beneficial. Supplementary information The online version contains supplementary material available at 10.1007/s10815-025-03573-8.

SARS-CoV-2, COVID-19, Live birth, Perinatal outcomes, Frozen-thawed embryo transfer

COVID-19, caused by SARS-CoV-2 infection, has swept the globe for over 5 years. As SARS-CoV-2 continues to evolve, COVID-19 waves are expected to remain common in the coming years, posing ongoing challenges to public health in various aspects [1–3]. Although the acute severity of COVID-19 has diminished, infections and reinfections remain prevalent worldwide [1, 2, 4, 5]. According to CDC data on COVID-19 Nucleic Acid Amplification Test (NAAT) percent positivity in the USA, cases appear to peak every 4 to 6 months, with weekly test positivity rates reaching as high as 17.6% during these surges [6]. Beyond its immediate impact on the respiratory system, COVID-19 has also affected reproductive health and global fertility [7–9]. Studies have shown that SARS-CoV-2 infection during pregnancy is associated with an increased risk of maternal and neonatal complications, including preterm birth, gestational diabetes, preeclampsia, and low birth weight, with severe cases even leading to stillbirths and maternal mortality [10–13]. For people recovering from COVID-19, the emergence of post-COVID-19 conditions, also known as “long COVID,” has introduced new variables into reproductive planning [14–16]. Furthermore, fertility treatments for infertile couples, especially medically assisted reproductive technologies (ART), have been significantly disrupted due to the pandemic. Many countries have witnessed a transient reduction in ART cycles and ART-related live births during this period due to the pandemic and its broader socioeconomic effects, potentially exacerbating the already existing declines in total fertility rates [17–21]. These facts highlight existing concerns and uncertainties regarding the safety of pregnancy with SARS-CoV-2 infection, even after recovery [21–23]. The setting of in vitro fertilization (IVF) and frozen-thawed embryo transfer (FET) actually provides an excellent model to study the association between SARS-CoV-2 infection and pregnancy outcomes in depth. With accurately documented dates of oocyte retrieval and embryo transfer, it is possible to investigate separately infection prior to oocyte retrieval (potentially affecting the embryo) and infection after oocyte retrieval but before embryo transfer (potentially affecting the endometrium). With respect to the former, a growing body of evidence has suggested that SARS-CoV-2 infection has no detrimental effects on oocyte yield, fertilization, and embryo quality in fresh cycles [24–26]. However, data regarding the effects of prior SARS-CoV-2 infection on the endometrium and FET outcomes remains limited. Only four studies have been identified, all with small sample sizes (ranging from 26 to 136 patients with a history of SARS-CoV-2 infection), and focused solely on early pregnancy outcomes, while data on live birth and perinatal outcomes remain lacking [27–30]. Therefore, we aimed to evaluate the effects of SARS-CoV-2 infection on live birth and perinatal outcomes following subsequent FET and to further investigate whether the time interval between infection and embryo transfer influences pregnancy outcomes. Our goal was to provide better guidance and recommendations for those with prior SARS-CoV-2 infection and to determine the optimal timing for subsequent FET.

Study participants This retrospective cohort study was approved by the Ethics Committee of Ren Ji Hospital affiliated to Shanghai Jiao Tong University School of Medicine (No. LY2024-302-B) and was in compliance with privacy-act guidelines. A waiver of informed consent was granted due to its retrospective design. This study followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline. Data were retrieved from the electronic clinical database of the Reproductive Medicine Center of Ren Ji Hospital affiliated to Shanghai Jiao Tong University School of Medicine. All autologous FET cycles between November 1, 2022, and May 30, 2023, were reviewed for eligibility. Throughout the study period, for each patient, a comprehensive COVID-19 questionnaire regarding infection status and symptoms was completed on-site and updated at each clinic visit. All individuals were tested with SARS-CoV-2 nucleic acid or antigen every other day by professionals at designated testing sites or during clinic visits in accordance with COVID-19 control policies. Women who tested positive before the embryo transfer were designated as the infected group, while women undergoing FET during the same study period who were asymptomatic and tested negative were included as the control group (non-infected group). The exclusion criteria were as follows: (1) couples who were infected with SARS-CoV-2 before oocyte retrieval or after embryo transfer; (2) couples who were not sure about their infection status or those without detailed information on SARS-CoV-2 infection; (3) the diagnosis of uterine abnormalities or untreated hydrosalpinges; (4) egg-freezing cycles; and (5) cycles with no detailed information on pregnancy or perinatal outcomes. For women who received multiple FET cycles during the study period, only their first cycle was included in this study. IVF and embryo transfer procedures All couples had completed a standard fertility evaluation before treatment initiation. IVF and embryo transfer protocols (including ovarian stimulation, oocyte retrieval, laboratory, and FET procedures) were individually tailored based on physician’s recommendation and patient’s preference [31, 32]. Briefly, all women underwent FET in this study, where either a programmed regimen, a natural regimen, or a stimulated regimen was used for endometrial preparation. For the programmed regimen, oral administration of estrogen valerate at a dose of 4–8 mg daily was started on day 2–5 of the menstrual cycle. Luteal phase support (LPS) with vaginal progesterone gel at a dose of 90 mg daily and oral dydrogesterone 10 mg twice daily was added when the endometrial thickness was adequate. For the natural regimen, natural ovulation was monitored by luteinizing hormone (LH) levels and ultrasound monitoring. LPS was started from the day of ovulation with oral dydrogesterone at a dose of 10 mg 2–3 times daily. For the stimulated regimen, letrozole at a dose of 2.5 mg daily was administered starting on day 3–5 for 5 days. Follicle development was monitored from day 10 onward, and human menopausal gonadotropin (HMG) was added as necessary. Ovulation was triggered by human chorionic gonadotropin (hCG) either when the dominant follicle reached ≥ 17 mm or when the serum LH surge was detected. LPS with oral dydrogesterone at a dose of 10 mg 2–3 times daily was started 2 days after triggering. Cleavage embryos and blastocysts were transferred after 3 days or 5 days of LPS, respectively, with no more than two embryos being transferred in each cycle. All transferred cleavage-stage embryos had at least 6 cells and less than 20% fragmentation on day 3, while blastocysts were scored as ≥ 4BC according to the Gardner criteria on day 5 or 6. If pregnancy was confirmed, LPS was continued until 8–12 weeks of gestation. All conceptions were followed through the end of pregnancy, which was completed in March 2024. Outcomes The primary outcome was live birth, defined as the delivery of any number of newborns at ≥ 28 weeks of gestation with signs of life. Secondary outcomes included positive HCG (defined as detection of serum β-hCG ≥ 10 mIU/mL measured 15–17 days after embryo transfer), clinical pregnancy (defined as observation of a gestational sac by a transvaginal ultrasound scan around 29–35 days after embryo transfer), pregnancy loss (defined as pregnancies that result in a spontaneous or therapeutic abortion before 28 weeks of gestation), and obstetric and neonatal outcomes [33, 34]. Obstetrical outcomes included gestational diabetes mellitus (GDM, 10th revision of the International Statistical Classification of Diseases and Related Health Problems [ICD-10] code O24), pregnancy-induced hypertension (PIH, ICD-10 code O13), preeclampsia (ICD-10 code O14), preterm delivery (defined as delivery of a fetus at more than 28 weeks and less than 37 of gestational age, ICD-10 code O60.1) [33], preterm premature rupture of membrane (PPROM, ICD-10 code O42), placenta previa (ICD-10 code O44), placenta accreta (ICD-10 code O43.2), placental abruption (ICD-10 code O45), and postpartum hemorrhage (ICD-10 code O72). Neonatal outcomes included birthweight, low birth weight (LBW, defined as birthweight < 2500 g, ICD-10 code O36.5), macrosomia (defined as birthweight ≥ 4000 g, ICD-10 code O33.5, O36.6), small for gestational age (SGA, defined as birthweight 90th percentile of gender-specific birthweight reference at the same gestational week), and congenital anomalies (defined as structural or functional anomalies that occur during intrauterine life, ICD-10 code Q00-Q99). Obstetric and neonatal outcomes were routinely collected in our electronic clinical database based on medical records obtained from the electronic medical history system at our university hospital or from responsible obstetricians and/or pediatricians. Statistical analysis Normally distributed continuous variables were expressed as mean and standard deviation (SD), while non-normally distributed variables as median and interquartile range (IQR). Student’s test or the Wilcoxon rank-sum test was used for between-group comparisons as appropriate. Categorical variables were expressed with number (n) and percentages (%). Between-group differences were assessed by the Pearson χ2 test, with Fisher’s exact test for expected frequencies less than 5. Univariate and multivariate logistic regression models were fitted to evaluate the association between SARS-CoV-2 infection and pregnancy outcomes. Potential confounding factors including age, body mass index (BMI), prior gravidity, prior parity, diagnosis of endometriosis, ART treatments, and vaccination status were adjusted. Results were presented with odds ratio (OR) and 95% confidence interval (CI). Furthermore, according to the time interval from the date of infection to embryo transfer, the infected group was categorized into four subgroups (30 days or less, 31 to 60 days, 61 to 90 days, and 91 days or more), to investigate whether the time interval from SARS-CoV-2 infection to subsequent FET would affect pregnancy outcomes. A two-sided p-value of less than 0.05 was considered statistically significant. No adjustment was performed for multiplicity of secondary outcomes. All analyses were performed using SPSS 21.0 (SPSS Inc., Chicago, IL, USA).

Study participants A total of 1946 patients and FET cycles were included in this analysis (Fig. 1), of which 1081 were included in the infected group (mean [SD] age, 32.8 [4.1] years), while 865 were in the non-infected group (33.0 [3.9] years). The mean time interval from SARS-CoV-2infection to FET was 77.7 ± 31.4 days. The majority of SARS-CoV-2-positive cases were asymptomatic or mild, with no severe (requiring oxygen support) or critical cases (requiring intensive care) observed. Approximately 91.2% (986/1081) of infected patients developed fever, with 76.0% (749/986) exhibiting temperatures < 39 °C. As presented in Table 1, patients in the infected group were less likely to be nulligravid (57.1% vs. 63.8%) and nulliparous (88.8% vs. 92.0%), had a higher proportion of endometriosis (14.5% vs. 10.8%), and were more likely to have been vaccinated (52.5% vs. 42.0%) compared to the non-infected group. There were no between-group differences observed in other baseline demographics including age, BMI, anti-Müllerian hormone, ART indications, previous retrievals, and transfers. In addition, no between-group differences were observed in laboratory outcomes and FET cycle characteristics. Table 1. | Infected group (N = 1081) | Non-infected group (N = 865) | P-value | | |---|---|---|---| | Baseline demographics | ||| | Age, mean (SD), y | 32.8 ± 4.1 | 33.0 ± 3.9 | 0.380 | | BMI, mean (SD), kg/m2 | 21.9 ± 3.2 | 21.9 ± 3.2 | 0.677 | | Prior gravidity, No. (%) | 0.003 | || | 0 | 617 (57.1) | 552 (63.8) | | | 1 | 214 (19.8) | 162 (18.7) | | | ≥ 2 | 250 (23.1) | 151 (17.5) | | | Prior parity, No. (%) | 0.040 | || | 0 | 960 (88.8) | 796 (92.0) | | | 1 | 101 (9.3) | 61 (7.1) | | | ≥ 2 | 20 (1.9) | 8 (0.9) | | | Anti-Müllerian hormone, mean (SD), ng/ml | 4.5 ± 3.4 | 4.6 ± 3.5 | 0.206 | | ART indication, No. (%) a | ||| | Tubal factor | 488 (45.1) | 414 (47.9) | 0.232 | | Ovulatory dysfunction | 205 (19.0) | 185 (21.4) | 0.185 | | Diminished ovarian reserve | 135 (12.5) | 93 (10.8) | 0.236 | | Endometriosis | 157 (14.5) | 93 (10.8) | 0.013 | | Male factor | 562 (52.0) | 420 (48.6) | 0.132 | | Unexplained b | 58 (5.4) | 45 (5.2) | 0.873 | | Recurrent pregnancy loss | 94 (8.7) | 61(7.1) | 0.183 | | Recurrent implantation failure | 77 (7.1) | 51 (5.9) | 0.278 | | Karyotype abnormality | 48 (4.4) | 44 (5.1) | 0.504 | | Monogenic disease | 23 (2.1) | 13 (1.5) | 0.309 | | Previous oocyte retrievals, median (IQR) | 0 (0–1) | 0 (0–1) | 0.578 | | Previous embryo transfers, median (IQR) | 0 (0–1) | 0 (0–1) | 0.885 | | Vaccination status, No. (%) | < 0.001 | || | Non-vaccinated | 513 (47.5) | 502 (58.0) | | | Vaccinated | 568 (52.5) | 363 (42.0) | | | Laboratory outcomes | ||| | ART treatment, No. (%) | 0.139 | || | IVF | 523 (48.4) | 449 (51.9) | | | ICSI | 313 (29.0) | 250 (28.9) | | | PGT | 245 (22.7) | 166 (19.2) | | | Oocytes retrieved, median (IQR) | 11 (7–17) | 12 (7–17) | 0.129 | | Good-quality embryos on day 3, median (IQR) | 5 (3–7) | 4 (3–8) | 0.861 | | Total viable embryos, median (IQR) | 4 (3–7) | 4 (3–7) | 0.757 | | Frozen-thawed embryo transfer | ||| | Endometrial preparation, No. (%) | 0.962 | || | Natural | 221 (20.4) | 180 (20.8) | | | Programmed | 771 (71.3) | 612 (70.8) | | | Stimulated | 89 (8.2) | 73 (8.4) | | | Endometrial thickness, mean (SD), mm | 9.0 ± 1.7 | 9.1 ± 1.8 | 0.537 | | Number of embryos transferred, mean (SD) | 1.2 ± 0.4 | 1.2 ± 0.4 | 0.294 | | Stage of embryos transferred, No. (%) | 0.848 | || | Cleavage | 272 (21.3) | 211 (21.0) | | | Blastocyst | 1006 (78.7) | 796 (79.0) | ART assisted reproductive technology, BMI body mass index (calculated as weight in kilograms divided by the square of the height in meters), ICSI intracytoplasmic sperm injection, IVF in vitro fertilization, PGT preimplantation genetic testing aCould have one or more indications to receive assisted reproductive technologies bUnexplained infertility is defined as the absence of identifiable causes for the infertility Pregnancy outcomes Positive HCG rate was significantly lower in the infected group than in the non-infected group (62.9% vs. 67.5%, P = 0.034), and results remained similar both before and after adjustment (crude OR = 0.82, 95% CI = 0.68–0.99; adjusted OR = 0.80; 95% CI = 0.66–0.97) (Table 2). Rates of clinical pregnancy (54.7% vs. 58.2%), pregnancy loss (26.2% vs. 24.8%) and live birth (45.9% vs. 49.7%) did not differ between groups. Results of logistic regression models, both unadjusted and adjusted, also revealed no associations between SARS-CoV-2infection with regard to clinical pregnancy, pregnancy loss, and live birth. Table 2. | No. (%) | Infected group (N = 1081) | Non-infected group (N = 865) | Crude OR (95% CI) | P-value | Adjusted OR a (95% CI) | P-value | |---|---|---|---|---|---|---| | Positive HCG | 680 (62.9) | 584 (67.5) | 0.82 (0.68 to 0.99) | 0.034 | 0.80 (0.66 to 0.97) | 0.021 | | Clinical pregnancy | 591 (54.7) | 503 (58.2) | 0.87 (0.73 to 1.04) | 0.124 | 0.85 (0.71 to 1.03) | 0.092 | | Pregnancy loss | 178/680 (26.2) | 145/584 (24.8) | 1.07 (0.83 to 1.38) | 0.584 | 1.07 (0.82 to 1.38) | 0.619 | | Among positive HCGs | 84/680 (12.0) | 74/584 (12.7) | 0.97 (0.83 to 1.38) | 0.865 | 0.96 (0.68 to 1.35) | 0.809 | | Among clinical pregnancies | 94/591 (15.9) | 71/503 (14.1) | 1.15 (0.82 to 1.61) | 0.410 | 1.14 (0.81 to 1.61) | 0.447 | | First trimester | 81/591 (13.7) | 59/503 (11.7) | 1.20 (0.84 to 1.71) | 0.330 | 1.18 (0.82 to 1.70) | 0.382 | | Second trimester | 13/591 (2.2) | 12/503 (2.4) | 0.92 (0.42 to 2.04) | 0.837 | 0.96 (0.43 to 2.14) | 0.911 | | Live birth | 496 (45.9) | 430 (49.7) | 0.86 (0.72 to 1.03) | 0.093 | 0.84 (0.70 to 1.01) | 0.062 | CI confidence interval, OR odds ratio aData were adjusted for age, body mass index, gravidity, parity, endometriosis, vaccination status, and assisted reproductive technology treatment Subgroup analysis Of the 1081 patients in the infected group, 82 were infected 0–30 days before embryo transfer, 211 were infected 31–60 days before embryo transfer, 450 were infected 61–90 days before embryo transfer, while 338 were infected > 90 days before embryo transfer. The mean time interval from SARS-CoV-2infection to FET was 16.1 ± 10.2, 52.0 ± 6.5, 75.1 ± 8.5, and 112.1 ± 22.0 days for the ≤ 30, 31–60, 61–90, and ≥ 91 days’ subgroup, respectively. A significant decrease in positive HCG rate was noted in the ≤ 30 days’ subgroup (53.7% vs. 67.5%, adjusted OR = 0.52, 95% CI = 0.33–0.84, P = 0.007) compared to the non-infected group, while no such association was observed for the 31–60, 61–90, and ≥ 91 days’ subgroups (Fig. 2). This pattern was also seen with respect to clinical pregnancy and live birth. Women in the ≤ 30 days’ subgroup had 49% decreased odds in achieving a live birth compared to those in the non-infected group (adjusted OR = 0.51, 95% CI = 0.31–0.83, P = 0.006), while no such association was noticed for the other subgroups. Obstetric and neonatal outcomes Table 3 details the incidence of obstetric and neonatal complications stratified by singleton and twin live births. Overall, no differences were observed in any of the measured outcomes among singletons and twins, respectively. Details of congenital anomalies are provided in Supplementary Table S1. Furthermore, no increased risk of obstetric and neonatal outcomes was observed across the ≤ 30, 31–60, 61–90, and ≥ 91 days’ subgroups (Supplementary Table S2 and S3). Table 3. | Outcome, No. (%) | Singleton | Twin | |||| |---|---|---|---|---|---|---| | Infected | Non-infected | P-value | Infected | Non-infected | P-value | | | Number of live birth cycles | 444 | 407 | 52 | 23 | || | Gestational diabetes mellitus | 66 (14.9) | 70 (17.2) | 0.353 | 9 (17.3) | 3 (13.0) | 0.745 | | Pregnancy-induced hypertension | 33 (7.4) | 20 (4.9) | 0.129 | 5 (9.6) | 3 (13.0) | 0.695 | | Pre-eclampsia | 19 (4.3) | 16 (3.9) | 0.798 | 7 (13.5) | 3 (13.0) | > 0.999 | | Preterm delivery | 32 (7.2) | 28 (6.9) | 0.852 | 30 (57.7) | 16 (69.6) | 0.330 | | Preterm premature rupture of membranes | 6 (1.4) | 6 (1.5) | 0.879 | 5 (9.6) | 2 (8.7) | > 0.999 | | Placenta previa | 0 (0) | 1 (0.1) | 0.478 | 0 (0) | 1 (4.3) | 0.307 | | Placenta accreta | 16 (3.6) | 9 (2.2) | 0.230 | 0 (0) | 0 (0) | NA | | Placenta abruption | 30 (6.8) | 23 (5.7) | 0.505 | 5 (9.6) | 1 (4.3) | 0.660 | | Postpartum hemorrhage | 9 (2.0) | 8 (2.0) | 0.949 | 2 (3.8) | 1 (4.3) | > 0.999 | | Number of live newborns | 444 | 407 | 103 a | 46 | || | Birthweight, mean (SD), g | 3321.0 (473.8) | 3300.4 (471.8) | 0.502 | 2439.7 (436.9) | 2525.2 (384.1) | 0.254 | | Low birth weight | 22 (5.0) | 16 (3.9) | 0.470 | 50 (48.5) | 22 (47.8) | 0.935 | | Macrosomia | 30 (6.8) | 22 (5.4) | 0.411 | 0 (0) | 0 (0) | NA | | Small for gestational age | 24 (5.4) | 20 (4.9) | 0.746 | 4 (3.9) | 2 (4.3) | > 0.999 | | Large for gestational age | 69 (15.5) | 56 (13.8) | 0.463 | 8 (7.8) | 8 (17.4) | 0.092 | | Congenital anomalies | 15 (3.4) | 10 (2.5) | 0.427 | 8 (7.8) | 3 (6.5) | > 0.999 | NA not applicable aOne woman with a twin pregnancy experienced a stillbirth of one twin during delivery

Our results showed that, overall, prior SARS-CoV-2 infection does not affect live birth rate, obstetric, and neonatal outcomes in women who received FET with unexposed embryos. However, a significant reduction in pregnancy outcomes was observed among women who received FET within 30 days after SARS-CoV-2 infection, suggesting that patients may benefit from delaying their subsequent FET for at least 30 days after the infection. There have been limited evidence regarding the effects of COVID-19 on subsequent FET cycles, and the findings remain controversial. In a retrospective cohort study, Youngster et al. reported that, in 41 women with SARS-CoV-2 infection and 41 matching controls, women with prior SARS-CoV-2 infection had lower clinical pregnancy rates (29.3% vs. 48.8%, P = 0.07) [27]. In contrast, Aizer et al. retrospectively compared 44 cycles from 26 post-infection patients to 125 cycles from 93 non-immune patients and reported no differences in clinical (29.5% vs. 26.4%) and ongoing (27.3% vs. 22.7%) pregnancy rates [28]. More recently, two observational studies—one retrospective study that included 110 infected patients and 88 uninfected controls and one prospective study involving 136 infected patients and 94 controls—both concluded that SARS-CoV-2 infection in women had no significant effect on clinical pregnancy rates following FET treatment [29, 30]. With a much larger sample size, our findings provide robust and reassuring evidence that, overall, prior SARS-CoV-2 infection in females does not appear to adversely affect live birth—except when FET occurs within 30 days of infection—or obstetric and neonatal outcomes following FET. However, the ideal time interval between SARS-CoV-2 infection and FET has not been fully understood. Youngster et al. reported significantly decreased clinical pregnancy rates in women who received FET within 60 days of recovery (n = 29) compared to matching controls (20.7% vs. 55.2%, P = 0.006) [27]. Comparatively, Huang et al. noted that the odds for clinical pregnancy were significantly lower in the ≤ 30 days subgroup (n = 15, adjusted OR = 0.15, 95% CI = 0.03–0.77), but not in the 31–60 days subgroup (n = 31) compared to the uninfected group [30]. Our study categorized the infected group on a month-by-month basis and discovered that women who underwent FET within 30 days of SARS-CoV-2 infection experienced lower pregnancy rates, whereas no such reduction was observed in those with longer intervals between infection and FET, suggesting an optimal window of > 30 days for a subsequent FET after SARS-CoV-2 infection. The underlying etiology behind this effect remains unknown. Previous research has shown that human endometria express ACE2 and TMPRSS2 transcripts and proteins, with higher ACE2 expression detected during the secretory phase [35–37], implying that the endometrium is theoretically vulnerable to SARS-CoV-2 infection, and that COVID-19 may potentially affect embryo implantation and early pregnancy outcomes. Additionally, COVID-19 has been shown to have a transient impact on menstrual patterns, including changes in menstrual volume and cycle duration in women [38], and in line with our findings, these abnormalities usually return to normal within 1–2 months after recovery [39]. This could partially explain why women who underwent FET within 30 days of SARS-CoV-2 infection had decreased pregnancy rates. Although numerous studies have demonstrated that SARS-CoV-2 infection during pregnancy increases the risk of maternal and neonatal morbidities, such as preterm delivery and low birth weight [40–42], our study demonstrated that a prior SARS-CoV-2 infection before FET did not adversely impact obstetric and neonatal outcomes. This may suggest that despite the presence of ACE2 and TMPRSS2 receptors in the endometrium, which may facilitate SARS-CoV-2 entry, it does not appear to cause lasting damage to the endometrial tissue. These findings offer reassurance, especially in light of the widespread concerns about the potential long-term consequences of COVID-19 on fertility treatments and subsequent pregnancies. To the best of our knowledge, this report represents the largest cohort study evaluating the effects of prior SARS-CoV-2 infection in FET cycles. It is also the first to report live birth and perinatal outcomes in this context. The low loss to follow-up rate (< 1%) and the comprehensive detailing of obstetric and neonatal outcomes enabled us to gather extensive information on the safety of FET and pregnancy in patients with a history of COVID-19. Additionally, by including only unexposed embryos in the FET cycles, our study minimized the potential influence of embryo factors during the process of implantation, allowing for a more focused investigation of the effects of COVID-19 on the endometrium and future pregnancy. Nonetheless, several limitations should be noted for this study. First, despite adjusting for multiple potential confounding factors, the possibility of selection bias should not be ignored due to the retrospective nature of this study. Second, while only asymptomatic patients with negative tests were included in the non-infected group, it is worth noting that SARS-CoV-2 antibodies were not tested for every woman. Residual bias regarding false-negative tests cannot be completely ruled out. Third, although the overall sample size is substantial for the infected group, the sample size for the subgroup with 30 days or less was relatively small, which limited the power to draw solid conclusions. The analyses of perinatal outcomes, particularly for twin pregnancies, were also underpowered due to limited cases. Additionally, while we examined multiple pregnancy and perinatal outcomes, we did not adjust for multiple comparisons in secondary analyses. Therefore, these results should be interpreted with caution. Large-scale prospective studies are warranted to verify our results.

Findings of this cohort study suggest that, overall, a SARS-CoV-2 infection prior to FET does not adversely affect live birth and perinatal outcomes. However, delaying the subsequent FET for at least 30 days after infection could be beneficial, considering a decreased likelihood of achieving live birth was observed among women with shortened intervals. These findings can guide clinical practices and alleviate concerns among patients and healthcare providers regarding the safety of proceeding with FET and pregnancy following COVID-19 recovery. Supplementary information Below is the link to the electronic supplementary material.

The authors would like to thank the clinicians, nurses, and laboratory staff in the Department of Reproductive Medicine, Ren Ji Hospital, Shanghai Jiao Tong University School of Medicine for their contributions. They received no compensation for their contributions. Author contribution YL, ZW, and YS were involved in the study concept and design. YH, JH, QZ, YD, and WL were involved in data collection. YL, ZW, and SL analyzed the data and drafted the manuscript. YS had a primary responsibility for final content. All authors were involved in the interpretation of the data, provided critical input to the manuscript, and approved the final manuscript. Funding This work was supported by the National Key R&D Program of China [2023YFC2705500, 2019YFA0802604], National Natural Science Foundation of China [82130046, 82320108009, 82371660, 32300710], Shanghai leading talent program, Innovative research team of high-level local universities in Shanghai [SHSMU-ZLCX20210200, SHSMU-ZLCX20210201, SSMU-ZLCX20180401], Shanghai Jiao Tong University School of Medicine Affiliated Renji Hospital Clinical Research Innovation Cultivation Fund Program [RJPY-DZX-003], Science and Technology Commission of Shanghai Municipality [23Y11901400], Shanghai’s Top Priority Research Center Construction Project [2023ZZ02002], and Three-Year Action Plan for Strengthening the Construction of the Public Health System in Shanghai [GWVI-11.1–36, GWVI-11.2-YQ21]. The funders had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication. Data availability The data that support the findings of this study are available from the corresponding author upon reasonable request. Declarations Ethics approval and consent to participate This retrospective cohort study was approved by the Ethics Committee of Ren Ji Hospital affiliated to Shanghai Jiao Tong University School of Medicine (No. LY2024-302-B) and was in compliance with privacy-act guidelines. A waiver of informed consent was granted due to its retrospective design. Conflict of interest The authors declare no competing interests. Footnotes Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Yao Lu and Zhe Wei contributed equally to this work.

- 1.de Anda-Jáuregui G, Gómez-Romero L, Cañas S, Campos-Romero A, Alcántar-Fernández J, Cedro-Tanda A. COVID-19 reinfections in Mexico City: implications for public health. Front Public Health. 2023;11: 1321283. [DOI] [PMC free article] [PubMed] [Google Scholar] - 2.Cai J, Zhang H, Zhu K, Zhu F, Wang Y, Wang S, et al. Risk of reinfection and severity with the predominant BA.5 Omicron subvariant China, from December 2022 to January 2023. Emerg Microbes Infect. 2024;13(1): 2292071. [DOI] [PMC free article] [PubMed] [Google Scholar] - 3.Mahase E. Covid-19: are we seeing a summer wave? BMJ. 2024;386: q1496. [DOI] [PubMed] [Google Scholar] - 4.Sánchez-de Prada L, Martínez-García AM, González-Fernández B, Gutiérrez-Ballesteros J, Rojo-Rello S, Garcinuño-Pérez S, et al. Impact on the time elapsed since SARS-CoV-2 infection, vaccination history, and number of doses, on protection against reinfection. Sci Rep. 2024;14(1):353. [DOI] [PMC free article] [PubMed] [Google Scholar] - 5.World Health Organization COVID-19 dashboard. 2024 [updated 31 March 2024. Available from: https://data.who.int/dashboards/covid19/cases?n=c. - 6.Centers for Disease Control and Prevention. COVID Data Tracker. 2025 [updated 03 May 2025. Available from: https://covid.cdc.gov/covid-data-tracker/#trends_weeklydeaths_testpositivity_00. - 7.Maher M, Owens L. SARS-CoV-2 infection and female reproductive health: a narrative review. Best Pract Res Clin Endocrinol Metab. 2023;37(4): 101760. [DOI] [PMC free article] [PubMed] [Google Scholar] - 8.Ata B, Vermeulen N, Mocanu E, Gianaroli L, Lundin K, Rautakallio-Hokkanen S, et al. SARS-CoV-2, fertility and assisted reproduction. Hum Reprod Update. 2023;29(2):177–96. [DOI] [PMC free article] [PubMed] [Google Scholar] - 9.Adelman S, Charifson M, Seok E, Mehta-Lee SS, Brubaker SG, Liu M, et al. State-specific fertility rate changes across the USA following the first two waves of COVID-19. Hum Reprod. 2023;38(6):1202–12. [DOI] [PMC free article] [PubMed] [Google Scholar] - 10.Villar J, Ariff S, Gunier RB, Thiruvengadam R, Rauch S, Kholin A, et al. Maternal and neonatal morbidity and mortality among pregnant women with and without COVID-19 infection: the INTERCOVID multinational cohort study. JAMA Pediatr. 2021;175(8):817–26. [DOI] [PMC free article] [PubMed] [Google Scholar] - 11.Hughes BL, Sandoval GJ, Metz TD, Clifton RG, Grobman WA, Saade GR, et al. First- or second-trimester SARS-CoV-2 infection and subsequent pregnancy outcomes. Am J Obstet Gynecol. 2023;228(2):226.e1-.e9. [DOI] [PMC free article] [PubMed] - 12.Gulersen M, Rochelson B, Shan W, Wetcher CS, Nimaroff M, Blitz MJ. Severe maternal morbidity in pregnant patients with SARS-CoV-2 infection. Am J Obstet Gynecol MFM. 2022;4(4): 100636. [DOI] [PMC free article] [PubMed] [Google Scholar] - 13.Veerus P, Nõmm O, Innos K, Allvee K, Karro H. SARS-CoV-2 infection during pregnancy and perinatal outcomes in Estonia in 2020 and 2021: a register-based study. Acta Obstet Gynecol Scand. 2024;103(2):250–6. [DOI] [PMC free article] [PubMed] [Google Scholar] - 14.Kandemir H, Bülbül GA, Kirtiş E, Güney S, Sanhal CY, Mendilcioğlu İ. Evaluation of long-COVID symptoms in women infected with SARS-CoV-2 during pregnancy. Int J Gynaecol Obstet. 2024;164(1):148–56. [DOI] [PubMed] [Google Scholar] - 15.Muñoz-Chápuli Gutiérrez M, Prat AS, Vila AD, Claverol MB, Martínez PP, Recarte PP, et al. Post-COVID-19 condition in pregnant and postpartum women: a long-term follow-up, observational prospective study. EClinMed. 2024;67: 102398. [DOI] [PMC free article] [PubMed] [Google Scholar] - 16.Davis HE, McCorkell L, Vogel JM, Topol EJ. Long COVID: major findings, mechanisms and recommendations. Nat Rev Microbiol. 2023;21(3):133–46. [DOI] [PMC free article] [PubMed] [Google Scholar] - 17.Gromski PS, Smith A, Lawlor DA, Sharara FI, Nelson SM. 2008 financial crisis versus 2020 economic fallout: how COVID-19 might influence fertility treatment and live births. Reprod Biomed Online. 2021;42(6):1087–96. [DOI] [PubMed] [Google Scholar] - 18.Scaravelli G, Levi Setti PE, Gennarelli G, Mencaglia L, Ubaldi FM, De Luca R, et al. The actual impact of SARS-CoV-2/COVID-19 pandemic on IVF activity: a survey across Italian ART centers. J Assist Reprod Genet. 2022;39(10):2373–80. [DOI] [PMC free article] [PubMed] [Google Scholar] - 19.Katagiri Y, Jwa SC, Kuwahara A, Iwasa T, Ono M, Kato K, et al. Assisted reproductive technology in Japan: a summary report for 2020 by the Ethics Committee of the Japan Society of Obstetrics and Gynecology. Reprod Med Biol. 2023;22(1): e12494. [DOI] [PMC free article] [PubMed] [Google Scholar] - 20.Azcorra H, Salazar-Rendón JC, Rodríguez L, Vázquez-Vázquez A, Mendez-Dominguez N. The impact of COVID-19 on the number of births in Yucatan, Mexico. Am J Hum Biol. 2023;35(4): e23849. [DOI] [PMC free article] [PubMed] [Google Scholar] - 21.Kahn LG, Trasande L, Liu M, Mehta-Lee SS, Brubaker SG, Jacobson MH. Factors associated with changes in pregnancy intention among women who were mothers of young children in New York City following the COVID-19 outbreak. JAMA Netw Open. 2021;4(9): e2124273. [DOI] [PMC free article] [PubMed] [Google Scholar] - 22.Gürtin ZB, Jasmin E, Da Silva P, Dennehy C, Harper J, Kanjani S. Fertility treatment delays during COVID-19: profiles, feelings and concerns of impacted patients. Reprod Biomed Soc Online. 2022;14:251–64. [DOI] [PMC free article] [PubMed] [Google Scholar] - 23.Venkatesh A, Boots CE, Anderson J, Anderson D, Citro L, Lawson AK. Patients’ views of delayed fertility care during the COVID-19 pandemic as a conception catastrophe: the experience of U.S. FertilityIQ users. J Psychosom Obstet Gynaecol. 2022;43(2):198–204. [DOI] [PubMed] [Google Scholar] - 24.Youngster M, Avraham S, Yaakov O, Landau Rabbi M, Gat I, Yerushalmi G, et al. Ivf under COVID-19: treatment outcomes of fresh ART cycles. Hum Reprod. 2022;37(5):947–53. [DOI] [PMC free article] [PubMed] [Google Scholar] - 25.Chen X, Shi H, Li C, Zhong W, Cui L, Zhang W, et al. The effect of SARS-CoV-2 infection on human embryo early development: a multicenter prospective cohort study. Sci China Life Sci. 2023;66(7):1697–700. [DOI] [PMC free article] [PubMed] [Google Scholar] - 26.Yang T, Wu L, Peng J, Wang C, Li G, Zhang J, et al. Effects of SARS-CoV-2 infection during ovarian stimulation on ART outcomes. Reprod Biomed Online. 2024;48(2): 103422. [DOI] [PubMed] [Google Scholar] - 27.Youngster M, Avraham S, Yaakov O, Landau Rabbi M, Gat I, Yerushalmi G, et al. The impact of past COVID-19 infection on pregnancy rates in frozen embryo transfer cycles. J Assist Reprod Genet. 2022;39(7):1565–70. [DOI] [PMC free article] [PubMed] [Google Scholar] - 28.Aizer A, Noach-Hirsh M, Dratviman-Storobinsky O, Nahum R, Machtinger R, Yung Y, et al. The effect of coronavirus disease 2019 immunity on frozen-thawed embryo transfer cycles outcome. Fertil Steril. 2022;117(5):974–9. [DOI] [PMC free article] [PubMed] [Google Scholar] - 29.Guo H, Yin M, Liu Y, Wang B, Lin J, Zhu Q. COVID-19 infection after oocyte retrieval did not have detrimental effects on embryo implantation for frozen embryo transfer. J Med Virol. 2023;95(8): e29054. [DOI] [PubMed] [Google Scholar] - 30.Huang J, Liu Y, Wang J, Xu D, Huang Z, Li M, et al. Pregnancy outcomes after frozen-thawed embryo transfer in women with COVID-19 history: a prospective cohort study. J Med Virol. 2024;96(1): e29377. [DOI] [PubMed] [Google Scholar] - 31.Zhang C, Yao N, Lu Y, Ni J, Liu X, Zhou J, et al. Ambient air pollution on fecundity and live birth in women undergoing assisted reproductive technology in the Yangtze River Delta of China. Environ Int. 2022;162: 107181. [DOI] [PubMed] [Google Scholar] - 32.Lu Y, He Y, Wang Y, Zhu Q, Qi J, Li X, et al. Effects of SARS-COV-2 infection during the frozen-thawed embryo transfer cycle on embryo implantation and pregnancy outcomes. Hum Reprod. 2024. 10.1093/humrep/deae068. [DOI] [PubMed] [Google Scholar] - 33.Clinical guidelines for the prevention and treatment of preterm birth (version 2024). Zhonghua Fu Chan Ke Za Zhi. 2024;59(4):257–69. [DOI] [PubMed] - 34.Chinese expert consensus on diagnosis and management of recurrent spontaneous abortion (2022). Zhonghua Fu Chan Ke Za Zhi. 2022;57(9):653–67. [DOI] [PubMed] - 35.Fernando SR, Chen X, Cheng KW, Wong BP, Qi S, Jiang L, et al. ACE inhibitors on ACE1, ACE2, and TMPRSS2 expression and spheroid attachment on human endometrial Ishikawa cells. Reprod Biol. 2022;22(3): 100666. [DOI] [PMC free article] [PubMed] [Google Scholar] - 36.Haouzi D, Entezami F, Tuaillon E, Gala A, Ferrières-Hoa A, Brouillet S, et al. SARS-CoV-2 and implantation window: gene expression mapping of human endometrium and preimplantation embryo. Life. 2021. 10.3390/life11121378. [DOI] [PMC free article] [PubMed] [Google Scholar] - 37.Chadchan SB, Popli P, Maurya VK, Kommagani R. The SARS-CoV-2 receptor, angiotensin-converting enzyme 2, is required for human endometrial stromal cell decidualization†. Biol Reprod. 2021;104(2):336–43. [DOI] [PMC free article] [PubMed] [Google Scholar] - 38.Khan SM, Shilen A, Heslin KM, Ishimwe P, Allen AM, Jacobs ET, et al. SARS-CoV-2 infection and subsequent changes in the menstrual cycle among participants in the Arizona CoVHORT study. Am J Obstet Gynecol. 2022;226(2):270–3. [DOI] [PMC free article] [PubMed] [Google Scholar] - 39.Li K, Chen G, Hou H, Liao Q, Chen J, Bai H, et al. Analysis of sex hormones and menstruation in COVID-19 women of child-bearing age. Reprod Biomed Online. 2021;42(1):260–7. [DOI] [PMC free article] [PubMed] [Google Scholar] - 40.Smith ER, Oakley E, Grandner GW, Ferguson K, Farooq F, Afshar Y, et al. Adverse maternal, fetal, and newborn outcomes among pregnant women with SARS-CoV-2 infection: an individual participant data meta-analysis. BMJ Glob Health. 2023. 10.1136/bmjgh-2022-009495. [DOI] [PMC free article] [PubMed] [Google Scholar] - 41.Litman EA, Yin Y, Nelson SJ, Capbarat E, Kerchner D, Ahmadzia HK. Adverse perinatal outcomes in a large United States birth cohort during the COVID-19 pandemic. Am J Obstet Gynecol MFM. 2022;4(3): 100577. [DOI] [PMC free article] [PubMed] [Google Scholar] - 42.Doyle TJ, Kiros GE, Schmitt-Matzen EN, Propper R, Thompson A, Phillips-Bell GS. Maternal and perinatal outcomes associated with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection during pregnancy, Florida, 2020–2021: a retrospective cohort study. Clin Infect Dis. 2022;75(Suppl 2):S308–16. [DOI] [PMC free article] [PubMed] [Google Scholar] Associated Data This section collects any data citations, data availability statements, or supplementary materials included in this article. Supplementary Materials Data Availability Statement The data that support the findings of this study are available from the corresponding author upon reasonable request.