Study population
The Generation R Study is a multi-ethnic, population-based, prospective cohort from fetal life onwards. This cohort has previously been described in detail.20, 21 Briefly, all pregnant women residing in Rotterdam with a planned delivery date between April 2002 and January 2006 were invited to participate and 9,778 (response rate 61%) women enrolled. The variable of interest, gestational age at antenatal care initiation, was available for 4,196 women. The Medical Ethics Committee of Erasmus MC in Rotterdam, the Netherlands, has approved the study in accordance with the Declaration of Helsinki of the World Medical Association. Written informed consent was obtained from all participants.