Is children's health up to two years of age affected by embryo vitrification?

Study question: Does embryo vitrification affect children's health including growth, up to 2 years of age when compared to fresh embryo transfer? Summary answer: While embryo vitrification had an impact on birth parameters, no differences in growth or health outcomes were found up to 2 years of age. What is known already: Vitrification has become the preferred cryopreservation method for embryos. Frozen embryo transfer has been repeatedly associated with altered health outcomes when compared with fresh transfer including a decreased risk for small-for gestational age (SGA) and an increased risk for large-for-gestational-age (LGA) and macrosomia. Not only there is uncertainty which factors are responsible for the observed differences, also the heterogeneity among studies limits overall conclusions. Notwithstanding the observed differences at birth, little is known about growth and health of children born after embryo vitrification beyond birth while aberrant growth trajectories have been linked to cardiometabolic morbidity later in life. Study design, size, duration: This single-center cohort study compared anthropometry and health outcomes in singletons conceived after cleavage-stage or blastocyst-stage embryo vitrification with results after fresh embryo transfer between 2014 and 2018. Pregnancies after PGT, IVM, oocyte vitrification or oocyte/embryo donation were excluded. Eligible singletons living in Belgium and randomly selected for continued follow- up were invited for examination in our center at 2 months (infancy) and 2 years of age (early childhood). Participants/materials, setting, methods: Birth characteristics were available for 1237 and 2063 children born after embryo vitrification and fresh embryo transfer, respectively. Follow-up data were available for 582 and 757 children at 2 months and for 233 and 296 children at 2 years. Growth parameters were adjusted for neonatal, treatment and maternal characteristics. Subgroup analysis according to cycle regimen (HRT versus NC) and strategy (freeze-all versus previous fresh cycle) was performed. In addition, outcomes restricted to blastocysts are presented. Main results and the role of chance: Mothers giving birth to a child conceived after embryo vitrification presented more often with pregnancy-induced hypertensive disorders than controls (P