RETROSPECTIVE COMPARISON OF PREGNANCY OUTCOMES OF SINGLE FRESH AND FROZEN-WARMED BLASTOCYST TRANSFER: A 5-YEAR SINGLE CENTER EXPERIENCE

OBJECTIVE: To assess whether live birth rates and maternal/neonatal complications differed following single fresh or single frozen-warmed blas tocyst transfer. MATERIALS AND METHODS: This retrospective observational study focused on single fresh and warmed day-five blastocyst transfers between January 1st, 2014 and December 31st, 2018. During fresh controlled ovarian stimulation cycle (COS), single embryo transfer (SET) was considered according to the age of the patient (<35 years) and a favorable prognosis. Patients at risk of ovarian hyperstimulation syn drome (OHSS), or with premature progesterone rise, or non-optimal endome trial growth or those with supernumerary embryos from a previous COS cycle, received frozen embryo transfer (FET) cycles. Using a patient tailored approach, three main protocols were available: natural cycles (NC-FET), modified natural cycle (mNC-FET) and artificial replacement (AR-FET). The analysis was corrected for the following covariates: maternal age and paternal age at induction, maternal body mass index (BMI), smoking history, years of infertility, primary or secondary infertility, ovarian reserve expressed as Follicular Stimulating Hormone (FSH), Anti-M€ullerian Hormone (AMH), and Antral Follicular Count (AFC) and indication to treatment. RESULTS: A total of 4,613 single embryo transfers (646 fresh and 3,967 frozen) during the five-year period from January 1st, 2014 to December 31st , 2018. After correction for multiple confounding factors, no differences in live birth rate (LBR) were recorded between the 2 groups. Fresh embryo transfer (ET) yielded a significant increase in neonatal complications OR 2.15 (95% CI 1.20 – 3.86, p 0.010) and placenta previa occurrence OR 3.58 (95% CI 1.54-8.28, p 0.003) when compared to frozen embryo transfer (FET), 9 (4.29%) in fresh ET and 15 (1.24%) in FET. Singletons born after FET were significantly larger (100 grams larger) (p 0.001). In addition, lower intrauterine growth was observed with a higher prevalence in the fresh group (3.33%) with a significant OR 3.18 (95% CI 1.25-8.07. p 0.015). Of rele vance, fresh ET yielded a higher prevalence of singletons weighing below the 5th percentile, 12.9% compared to 7.4% in the frozen ET subgroup (p 0.013). CONCLUSIONS: Live birth rate (LBR) seems not to be affected by which embryo transfer procedure is chosen. In addition, fresh embryo transfer is associated with higher risk of neonatal complications (specifically intrauter ine growth retardation), placenta previa and a higher prevalence of singletons weighting below the 5th percentile. IMPACT STATEMENT: Considering the increased use of ART proced ures worldwide, it is important to understand whether fresh or frozen embryo transfers are associated with the best maternal and neonatal outcomes before proposing a ‘‘freeze-all’’ as standard practice for every infertile couple. Clinical Trial Registration Number: NCT04310761.