Bone mineralization in children aged 7-10 years born after ART with frozen and fresh embryo transfer.

Study question: Does height-adjusted bone mineral content (BMC) at 7-10 years of age differ between children conceived after ART with frozen embryo transfer (FET) compared to children conceived after ART with fresh embryo transfer (fresh-ET) or naturally conceived (NC) children?

Summary answer: Children conceived after FET had an increased BMC corrected for height compared with fresh-ET and NC when adjusted for confounders, a difference primarily mediated by the higher birthweight (BW) found in children conceived after FET compared with fresh-ET and NC.

What is known already: Children conceived after ART with FET have a higher BW compared to NC children, while the opposite association is known for fresh-ET. In NC children, BW is positively associated with BMC and bone mineral density (BMD), but bone health in children born after ART is scarcely explored with inconsistent results.

Study design size duration: This study was a retrospective cohort study as part of the 'Health in Childhood following Assisted Reproductive Technology' (HiCART) cohort, consisting of 606 singletons (292 boys), conceived after FET (n = 200), fresh-ET (n = 203), and NC (n = 203) born from November 2009 to December 2013. The children were 7-10 years old when clinically examined, and the study was conducted from January 2019 to September 2021.

Participants/materials setting methods: Children were identified through the Danish Medical Birth Registry and Danish IVF Registry. These registries were also used to collect information regarding ART treatment, pregnancy, and delivery. The clinical examination involved anthropometric measurements, pubertal staging, fasting blood samples, and a whole-body dual-energy x-ray absorptiometry (DXA) scan. Both parents completed a questionnaire on background information regarding themselves and their child. The three groups were compared pairwise using univariate linear regression, and possible confounders and mediators were adjusted for using multiple linear regression analysis.

Main results and the role of chance: Crude values of BMC corrected for height did not differ between children born after FET, fresh-ET, or NC. When adjusted for relevant confounders, children born after FET had a statistically significant higher BMC corrected for height compared with both fresh-ET and NC. After further adjustment for BW SD score, the differences in BMC corrected for height disappeared, and no statistically significant differences in BMC corrected for height between any of the three groups were found. Factors potentially affecting bone mineralization, such as calcium, parathyroid hormone (PTH), insulin-like growth factor-1 (IGF-1), lean mass, and physical activity in childhood, did not differ between the three groups.

Limitations reasons for caution: All relevant confounders were adjusted for, although the cause of infertility was not available, hence a risk of residual confounding. The use of a parental questionnaire on the child's physical activity level sets a risk of information bias.

Wider implications of the findings: Increased BW in children conceived after FET was associated with increased BMC corrected for height at age 7-10 years when compared to children conceived after fresh-ET and NC. Longitudinal studies of pregnancies and newborns after FET are needed to explore the causes of the increase in BW and the possible effects on long-term bone health.

Study funding/competing interests: The study was funded by the Novo Nordisk Foundation (grant number: NNF18OC0034092, NFF19OC0054340) and Rigshospitalet Research Foundation. A.P. has received grants from Gedeon Richter, Ferring Pharmaceuticals, Merck, and Cryos; consulting fees from Gedeon Richter, Ferring Pharmaceuticals, Merck, Cryos, and IBSA; speaker fees from Gedeon Richter, Ferring Pharmaceuticals, Merck, and Organon; and travel grants from Gedeon Richter. The remaining authors declare no conflicts of interest.

Trial registration number: ClinicalTrials.gov identifier: NCT03719703.