Vasa Previa

Normally, the placenta implants high up in the uterus and the umbilical cord inserts into the center of the placenta. This leaves plenty of room for the fetus to enter the birthing position and keeps blood vessels in the umbilical cord out of harm's way.

However, some rare abnormalities can cause these fetal blood vessels to lie low and exposed in the uterus where they are in danger of tearing during vaginal labor.

First, velamentous insertion of the umbilical cord can cause vasa previa. Normally, the umbilical cord "inserts," or attaches, into the thickest, centermost part of the placenta. However, the cord will rarely insert into the amniotic sac instead of into the placenta.

With a velamentous cord insertion, the arteries and veins inside the umbilical cord, unprotected by umbilical tissue, run through the amniotic sac before connecting with the placenta. Vasa previa happens when those umbilical blood vessels run through the part of the amniotic sac above the cervix.

Secondly, a bilobed or succenturiate placenta can cause vasa previa. The placenta can form into two or more separate lobes, becoming bilobed or multilobed. A smaller, accessory lobe called a succenturiate lobe can also form. Fetal blood vessels traveling between these lobes may end up positioned above the cervix, causing vasa previa.

Multiple lobes can be formed due to the placenta's tendency to seek out blood-rich areas of the uterus. Certain areas in the uterus may be less vascular than others. In fact, movement of the placenta as it seeks out blood-rich areas of the uterus is also thought to be the cause of velamentous insertion.

Pregnancies in which one of the two umbilical arteries is missing or in which the mother is carrying twins are more likely to experience complications like these. Mothers who smoke, are more advanced in age, who have a chronic health condition like diabetes, or who conceived via in vitro fertilization are also at risk.

60% of mothers with vasa previa have a history of low-lying placentas or placenta previa, a condition in which the placenta implants near to or on top of the cervix.

Vasa previa is rare, occurring in only about .04% of pregnancies. If diagnosed, carefully observed, and delivered by c-section, vasa previa pregnancies almost always result in perfectly healthy babies.

However, vaginal delivery is overwhelmingly fatal to the baby in vasa previa pregnancies. With vasa previa, blood vessels are located in the fetal membranes, also known as the amniotic sac, and cross over the cervix. During delivery, the amniotic sac pops and the baby is forced down through the cervix, rupturing the blood vessels.

Since these vessels are attached to the baby via the umbilical cord, vaginal delivery results in the baby losing a large quantity of blood. This scenario can be completely avoided, however, with c-section delivery.

Another concern with vasa previa is that blood vessels can be pinched, cutting off circulation to the fetus. Lack of oxygen is a serious concern because it can result in brain damage.

It's possible for women with vasa previa to show no outward symptoms. However, they may experience painless vaginal bleeding. Blood that is darker in color is likely coming from the fetus since fetal blood is less oxygenated than maternal blood. Any signs of fetal distress, such as bleeding or a slow heart rate, should be detected and investigated by a doctor during prenatal checkups.