More than 3000 children or 5–6% of the Danish birth cohort are born annually after in-vitro fertilization (IVF) techniques. Frozen embryo transfer (FET) has been increasingly used to avoid twin gestations as a single embryo can be transferred and the surplus embryos frozen. Vitrification, an ultra-fast freezing method, has improved the survival of frozen embryos from < 50% with the slow-freeze method to > 95%. However, singletons born after FET have a higher risk of being born larger than others at same age, which may increase their risk of diabetes and cardiovascular disease. In this project we will examine BMI, metabolism, endocrine and cardiovascular parameters and their genome control mechanisms (epigenetic modification) in 600 singletons aged 6 to 8-years born after A. FET; B. Fresh embryo transfer; and C. Natural conception. This study is the first of its kind worldwide and will have wide implications for the IVF treatment strategies in the future (www.HiCART.dk).