“Katherine Bligard, MD, a WashU Medicine maternal-fetal medicine specialist at Barnes-Jewish Hospital and St. Louis Children’s Hospital who cared for Sara at the Fetal Care Center, explains that because of the difference in the placenta’s blood vessel connections, Baby A was dehydrated and Baby B was overly hydrated. “Amniotic fluid is baby’s pee,” Dr. Bligard says. “So if a baby’s dehydrated, they’re going to pee less, and so they make less amniotic fluid. Both dehydration and overhydration can make babies very sick in the uterus.”