Eligible women are recruited from the cohort enrolled to nuMoM2b. For nuMom2b a convenience sample was recruited through 8 primary clinical sites that NIH selected because they could each demonstrate access to more than 4,500 deliveries per year. As a group, sites were geographically dispersed and ethically diverse. All nulliparous women aged 13 years or greater with a singleton pregnancy less than 13 weeks 6 days gestation who presented at recruitment clinics or practices and planned to deliver at a study hospital were potentially eligible for participation. A clinical or research ultrasound, performed by a study certified sonographer, confirmed viability of a singleton gestation with no obvious malformations and estimated gestational age less than or equal to 13 weeks 6 days before the woman was allowed enrollment. The first visit was scheduled and completed within a window of 6 weeks 0 days and 13 weeks 6 days estimated gestational age.