FROZEN-THAWED EMBRYO TRANSFERS ARE ASSOCIATED WITH A HIGHER RISK OF LARGE FOR GESTATIONAL AGE NEWBORNS

Abstract

Improvements in cryopreservation techniques associated with the possible impairment in endometrial receptivity due to the supra-physiologic hormonal levels observed during conventional controlled ovarian stimulation (COS) have increased the implementation of frozen-thawed embryo transfers (FET). Perinatal outcomes of fresh embryo transfers (ET) and FET have shown to be similar. However, several studies associate FET with a higher risk of large-for-gestational-age (LGA), thereby, increasing risk of adverse obstetric and neonatal outcomes. This study aims to assess whether FET is associated with a higher risk of large for gestational age newborns, compared with fresh ET in our population. Retrospective, single center study performed from 2017 to 2019. A total of 561 patients who became pregnant after fresh and frozen embryo transfers. A first study group included all IVF cycles where fresh embryo transfer resulted in a singleton live birth (fresh group n=254). A second cohort included FET that led to a singleton live birth (FET group n= 307). All embryos were transferred at a blastocyst stage and vitrification was the only cryopreservation method. In all FET the endometrium was artificially prepared through the administration of exogenous estrogen and progesterone. Data was collected from telephone surveys. We compared the number of LGA newborns and weight differences in both groups. z-test was applied for statistical analysis. A p value <0.05 was considered significant. The median gestational age in both groups was 38 weeks. The mean adjusted birth weight after FET group was higher by 109.26 g, than the fresh group (3269.32 g vs 3160.13 g respectively, p=0.285). The incidence of newborns that weighted ≥ p50 was significantly higher for FET group (142 vs 75 for FET and fresh respectively, p= 0.000048). Moreover, the incidence of LGA livebirths was significantly higher in the FET group (p=0.0226) (23 from the FET group vs 8 from fresh ET). No difference in female to male ratio for LGA newborns or preterm birth rate was identified between groups. FET is associated with increased risk of LGA in our population. FET has become an important technique in IVF; however, whether it should be the first choice for ET requires further analysis. An individual approach should remain when deciding between fresh or frozen embryo transfers. Longer-term potential health effects remain to be evaluated.