Similar Perinatal Outcomes In Children Born After Fresh Or Frozen Embryo Transfer Using Donated Oocytes.

Singletons born after fresh embryo transfer (ET) are associated with higher rates of preterm birth and low birthweight, while frozen embryo transfers (FET) seem to convey a higher risk of large for gestational age. However, studies comparing these outcomes using autologous oocytes are unable to adequately disentangle the potential effect of embryo vitrification from the possible consequences on endometrial receptivity caused by ovarian stimulation/preparation, prior to ET. Hence, the oocyte donation (OD) model is optimal for this differentiation and, so far, information available regarding neonatal outcomes is limited to either small and/or heterogeneous studies. We performed a multicenter retrospective cohort study including 5848 singletons born between 2009 and February 2020 following OD and single blastocyst transfer, subdivided into fresh ET and FET groups. Patients with a first singleton livebirth after single blastocyst transfer were compared using multivariable regression analysis to account for potential confounding. The primary outcome was birthweight. Secondary outcomes were low birth weight (below 2500g and 1500g), birthweight z-scores, small/large for gestational age, prematurity, neonatal morbidity (Apgar scores and need for neonatal intensive care) and maternal morbidity (gestational hypertensive disorders, diabetes and cesarean delivery). The control variables included were female recipient/donor age, body mass index and smoking status, sperm source, endometrial thickness and preparation technique (natural or artificial cycle), serum estradiol and progesterone levels, and newborn gender. Continuous outcomes are presented with medians and interquartile ranges (IQRs) while dichotomous outcomes are shown using percentages and 95% confidence intervals (CIs). There was no significant difference between the fresh ET and FET groups in terms of birthweight (3215 g, IQR [2900g, 3540 g]; versus 3200.0g, IQR [2860g, 3500 g]) and birthweight z-scores (0.03, IQR [-0.67, 0.73]; versus 0.1, IQR [-0.59, 0.71]), in both the unadjusted and confounder-adjusted models. However, artificial endometrial preparation was associated with a higher birthweight (3220 g, IQR [2900g, 3540 g]; versus 3105 g, IQR [2800g, 3450 g]) and birthweight z-scores (0.06, IQR [-0.63, 0.74]; versus -0.13, IQR [-0.73, 0.59]), even following confounder adjustment. The premature birth rates (<37weeks) were, respectively, 9.9% (8.9%-10.8%) and 11.2% (9.8%-12.6%) for fresh ET and FET, while the very premature birth rates (<32 weeks) were 1.4% (1.0%-1.8%) and 1.9% (1.3%-2.5%), with no significant difference, even following confounder adjustment. There were also no statistically significant differences in other neonatal outcomes and maternal morbidity. Perinatal outcomes did not seem to be affected by the embryo vitrification process in an OD model. Other factors may contribute to the hindered perinatal outcomes described, particularly the potential effect ovarian stimulation and endometrial preparation may have on endometrial receptivity.