Impact of Embryo Cryopreservation on Large for Gestational Age Babies Born by Embryo Transfer: Cohort Retrospective Study.

The transfer of frozen-thawed embryos has been associated with an increased risk of large for gestational age at birth. Our objective is to assess its impact on the risk of large for gestational age (LGA) in order to improve the bias control in relation with the available studies. Retrospective cohort study on cumulative sample of 801 single live births of 32 weeks or longer of gestation, resulting from pregnancies obtained by cryopreserved or fresh embryo transfer which are not affected by disorders that could impair fetal growth and carried out at Hospital General Universitario Gregorio Marañón, in Madrid, during the period 2005-2017. The relative risk (RR) of LGA has been estimated with its confidence interval (CI) at 95%. Multivariate analysis using logistic regression was applied to adjust the crude effect. LGA was more frequent in babies born after cryotransfer in comparison with the reference group (20.9% vs. 6.0%; p < 0.001), as well as macrosomia (8.2% vs. 0.9%; p < 0.001). Frequencies of weight > 4500 g were similar (0.7% vs. 0.1%; p > 0.05). Nulliparity was associated to a higher risk of LGA (RR: 3.8; CI95%: 2.0-7.0; p < 0.005), as well as cleaving embryo transfer (RR: 2; CI95%: 1.07-3.8; p < 0.05). According to the multivariate analysis, the exposure variable was the only one independently associated with LGA (OR: 3.5; CI95%: 2.0-6.1; p < 0.001). Frozen-thawed embryo transfer significantly increases the risk of LGA, regardless of the influence of factors relating to the patient's condition, the embryos transferred, or the fetal sex.