Vasa Previa

Vasa previa is present when fetal blood vessels travel through the membranes within 2 cm of the internal cervical os. The vessels are not protected by the umbilical cord or placental tissue. When the membranes rupture, the exposed vessels may tear, producing rapid blood loss from the fetal circulation. That mechanism explains the striking clinical mismatch: the fetus can deteriorate rapidly while the pregnant patient’s blood pressure and pulse initially remain normal. The bleeding is fetal, not maternal. A small amount of vaginal blood paired with a sudden fetal heart rate change therefore deserves more urgency than the maternal blood loss alone might suggest. Risk is increased by velamentous cord insertion, a low-lying placenta, bilobate or succenturiate placental lobes, in-vitro fertilization, multiple gestation—particularly monochorionic twins—and a single-umbilical-artery pregnancy. These findings do not establish vasa previa, but they should prompt careful assessment of where the fetal vessels lie in relation to the cervix.