Objective: To explore obstetric and perinatal outcomes in singleton pregnancies occurring as a result of fresh and thawed frozen embryo transfer using anonymized Human Fertilisation and Embryology Authority data. Design: Retrospective cohort study. Setting: Not applicable. Patient(s): Singleton births after IVF/intracytoplasmic sperm injection cycles in the United Kingdom (1991-2011). Intervention(s): A total of 112,432 cycles (95,911 fresh and 16,521 frozen) were analyzed using multivariate logistic regression to explore associations between type of embryo transferred (frozen vs. fresh) and obstetric and perinatal outcomes. Relative risks (RRs) and their 99.5% confidence intervals (CIs) were calculated using Stata 14 MP, adjusting for potential confounders. Main Outcome Measure(s): Birth weight. Result(s): The adjusted RR (99.5% CI) of low birth weight [0.73 (0.66-0.80)] and very low birth weight [0.78 (0.63-0.96)] were all lower after frozen embryo transfer; however, RR of having a high birth weight baby was higher [1.64 (1.53-1.76)]. There was no difference in RR of preterm birth [0.96 (0.88-1.03)], very preterm birth [0.86 (0.70-1.05)], and congenital anomalies [0.86 (0.73-1.01)]. Conclusion(s): The findings of low birth and very low birth weight after thawed frozen embryo transfer are consistent with the literature and provide reassurance regarding the outcome of pregnancies after frozen embryo transfers. However, they highlight the possibility of high birth weight in these babies. Because these results are based on observational data, further evidence from randomized, controlled trials is needed before elective cryopreservation of all embryos is practiced in preference to the current practice of transfer of fresh embryos.
