In a healthy pregnancy, fetal blood vessels run through the umbilical cord, connecting the infant to the placenta. Vasa previa is a condition in which fetal vessels move out of the umbilical cord’s protection and travel through the membranes that lie across the birth canal. This can happen for one of these two reasons:
The cord implants in the fetal membranes instead of the placenta, and the fetal vessels need to get to the placenta.
The placenta is divided into two “lobes” and the vessels must connect the two.
Vasa previa is a dangerous condition because the unprotected fetal vessels may end up rupturing along with the amniotic sac. If this happens, it can lead to massive fetal blood loss.