Guided by ultrasound, your doctor pinpoints the spot where the umbilical cord meets the placenta. He then inserts a needle through your abdomen and uterus and into the umbilical cord and withdraws a small amount of fetal blood.
Update Date 6/11/2014
Updated by: Cynthia D. White, MD, Fellow American College of Obstetricians and Gynecologists, Group Health Cooperative, Bellevue, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.