The purpose of this study was to analyse the obstetric outcome after frozen embryo transfer depending on the protocol used. The endometrial preparation during frozen embryo transfer (FET) was performed in the natural cycle (NC) with timing based on monitoring of the naturally occurring luteinizing hormone (LH) peak or in human chorionic gonadotropin (hCG)-triggered modified natural cycles (MNC), artificial cycle (AC) with hormone replacement therapy cycle and cycle with ovulation induction (OI). A retrospective cohort study was conducted between 2018 and 2020. Pregnancy, perinatal complications and neonatal outcomes were compared among patients treated during AC or MNC or OI protocols. After adjusting for the effect of gravidity, parity, pre-pregnancy body mass index (BMI) and number of miscarriages, the AC group had higher but not significantly different rates of gestational hypertension, preeclampsia and intrahepatic cholestasis of pregnancy (ICP) than women in OI and MNC groups. Significant differences were observed in the rates of placental adherence (15.30% in AC vs. 9.24% in MNC, p = 0.004) and postpartum haemorrhage (PPH) (24.33% in AC vs. 13.07% in OI, p = 0.003, 24.33% in AC vs. 16.24% in MNC, p = 0.002) among the three groups. In singletons, significant differences were observed in the rates of placental adherence (14.09% in AC vs. 8.57% in MNC, p = 0.002), AC and MNC groups had higher risk of PPH compared with OI group (18.36% in AC vs. 12.38% in MNC, p = 0.042 and 7.69% in OI vs. 18.36% in AC, p = 0.013). Neonatal outcomes were similar among the three groups except risk of small for date infants (0.35% in AC vs. 1.44% in MNC, p = 0.008). These findings are important in view of increasing use of frozen cycles and new policy of freeze-all cycles in in vitro fertilization (IVF). The results suggest a link between the absence of corpus luteum and adverse obstetric outcomes.
Keywords: Assisted reproductive technology; endometrial preparation; frozen embryo transfer; neonatal outcomes; pregnancy complications.