The NICHD Fetal Growth Studies aimed to empirically define the predominant trajectory of fetal growth in twins using longitudinal two-dimensional ultrasound and to compare the twin fetal growth trajectories with the singleton growth standard. Pregnant women were recruited from 2009 to 2013 from 12 participating clinical sites across the United States of America. The total sample of almost 3,000 pregnant women was categorised into three cohorts. The first was a low-risk singleton cohort of over 2000 women within a normal to overweight BMI range. The second was a singleton cohort of around 470 women in the obese range of the BMI. The third was a cohort of around 170 women pregnant with twins. Baseline assessment occurred at 10 to 13 weeks gestation. The singleton cohort was followed up 5 times throughout their pregnancy, and the twin cohort was followed up six times. All participants were followed up at delivery, and a subset was followed up six weeks after birth.