Growth of singletons born after frozen embryo transfer until early adulthood: a Finnish register study

Abstract STUDY QUESTION Are there growth differences between singleton children born after frozen embryo transfer (FET), fresh embryo transfer (ET), and natural conception (NC)? SUMMARY ANSWER Adolescent boys born after FET have a higher mean proportion and increased odds of overweight compared to those born after fresh ET. WHAT IS KNOWN ALREADY Children born after FET have higher mean birthweights and an increased risk of large-for-gestational-age compared to those born after fresh ET and even NC. This raises questions about possible growth differences later in childhood. Previous studies on child growth after FET report partly conflicting results and lack long-term data until adolescence. STUDY DESIGN, SIZE, DURATION This was a cohort study based on national population-based registers, the Finnish Medical Birth Register and the Register of Primary Health Care visits, including singletons born after FET (n = 1825), fresh ET (n = 2933), and NC (n = 31 136) in Finland between the years 1995 and 2006. PARTICIPANTS/MATERIALS, SETTING, METHODS The proportions of overweight (i.e. age- and sex-adjusted ISO-BMI for children ≥ 25) were compared between the groups. Odds ratios (ORs) and adjusted odds ratios (aORs) of overweight were calculated. Adjustments were made for birth year, preterm birth, maternal age, parity, and socioeconomic status. Mean heights, weights, and BMIs were compared between the groups each year between the ages of 7 and 18. MAIN RESULTS AND THE ROLE OF CHANCE FET boys had a higher mean proportion of overweight (28%) compared to fresh ET (22%, P