Overview
Why Vessel Location Creates a Fetal Emergency
Vasa previa occurs when unprotected fetal blood vessels—arterial or venous—run through the membranes over or within 2 cm of the internal cervical os.
Vasa previa occurs when unprotected fetal blood vessels—arterial or venous—run through the membranes over or within 2 cm of the internal cervical os. Normally, umbilical vessels are protected by Wharton jelly within the cord or by placental tissue. In vasa previa, the vessels lie exposed in the membranes, where rupture of the membranes or an amniotomy can tear them. The consequence is fetal hemorrhage, not primarily maternal hemorrhage. A fetus has a small circulating blood volume, so even a seemingly modest amount of blood in the vagina may represent a life-threatening loss. The fetal heart-rate tracing can deteriorate within minutes while the pregnant patient’s blood pressure and pulse remain normal because her circulation has not lost that blood. Two anatomic patterns are commonly described. In one, a velamentous cord insertion leaves umbilical vessels traveling unprotected through the membranes before reaching the placenta. In the other, vessels connect the main placenta with a succenturiate or separate placental lobe and cross the lower uterine segment. The mechanism is the same: exposed fetal vessels are positioned where cervical dilation, membrane rupture, or instrumentation...