The NICHD Fetal Growth Study was a multi-site, prospective cohort study designed to establish a standard for normal fetal growth (velocity) and size for gestational age in the U.S. population. For the singleton study, 2,334 low-risk, non-obese gravidas were recruited from each of 4 self-identified race/ethnicity backgrounds. An additional 468 obese women and 171 dichorionic twin gestations were recruited. After an initial sonogram in the first trimester, women were randomized to serial ultrasonology schedules. Each schedule comprised five additional sonograms for fetal biometry (six in twins) plus additional image and 3D volume acquisition for later analysis.