Strengths and Limitations
To our knowledge, this is the first study assessing the combined contribution of maternal and paternal preconception overweight and obesity on infant birthweight. The strengths of our study include the use of a database in which preconception health, IVF cycle outcomes and pregnancy rates were registered prospectively, thereby minimising selection bias; BMI was calculated from clinically recorded measurements of maternal and paternal preconception weights and heights; the analysis only included first singleton term births; and the large population size from a singular ART unit limited variability in clinical protocols. Limitations of our study include the retrospective study design, which limits the degree of causal inference; reduced ability to control for some key parental factors that can influence infant birthweight, including parental smoking 15, 16 and maternal gestational weight gain 17 and further, the fact that the utilization of an ART cohort is confounded by subfertility andin vitro embryo culture. However, infertility diagnosis has been previously shown to not influence infant birthweight in term pregnancies18, 19, thus the subfertility diagnosis is unlikely to be contributing to the reported outcomes.