Cryopreserved Embryo Replacement is Associated with Higher Birthweight Compared with Fresh Embryo: Sibling Cohort Study

Abstract

Background: It is known that birth weight (BW) is higher after frozen embryo transfer (FET) than after fresh embryo replacement. No study has compared the BW of siblings conceived using the same oocyte/embryo cohort but different transfer procedures (fresh vs frozen). The aim of this study was to determine whether the freezing-thawing procedure itself is involved in such difference. Methods: Multicenter retrospective study at the ART/PGD Department of Montpellier University Hospital, Clinique Ovo, Canada and Grenoble-Alpes University Hospital. The first cohort (Fresh/FET) included all in vitro fertilization (IVF) cycles where the older child was born after fresh embryo transfer (fresh, n=158) and the younger one after transfer of frozen supernumerary embryos conceived during the same IVF cycle (FET, n=158). The second cohort (FET/FET) included all IVF cycles where both older and younger child were born after FET of embryos from the same cohort (FET1, n=25; FET2, n=25). Findings: The mean adjusted BW of the FET group was significantly higher than that of the fresh group (3508.9 452.4 g vs 3237.7 463.3 g; p<0.01). The FET group showed increased risk to be large for gestational age (AOR 4.22; 95%CI 2.04-8.73). In the FET/FET cohort, the mean adjusted BW was higher for the younger baby by 93.1 g but this difference is not significant (3430.2 347.6 g vs 3337.1 391.9 g; p=0.3789). Interpretation: As both embryos were from the same IVF cycle, the only difference was the cryopreservation step, our results strongly suggest that cryopreservation is directly involved in the BW variation. Moreoever, comparing BW difference between Fresh/FET cohort and the FET/FET one, our study suggests that parity is not the only responsible in this difference, increasing the role of cryopreservation step in BW variation. Funding: Partially financial support was covered by INSERM U 1203. Declaration of Interest: None of the authors has any conflicts of interest to declare. Ethical Approval: Written informed consent was obtained for the use of data in national registries, and the local Institutional Review Boards approved the study. All patients at the Ovo clinic signed a treatment consent form that included consenting to the use of non-identifying information for publication purposes.