A fetus growing in the womb is connected to its mother via the placenta and umbilical cord. The placenta is attached to the interior of the mother’s uterus and is linked to the fetus by the umbilical cord. These temporary organs, which are delivered along with the baby during childbirth, are full of blood vessels.
Inside the placenta, diffusion occurs between the mother’s blood vessels and the fetus’ blood vessels. This allows for food and nutrients to pass from the mother to the fetus and for waste products like carbon dioxide to pass from the fetus to the mother.
The umbilical cord consists of two arteries and a vein. Sometimes, these blood vessels lie across the opening of the cervix, the opening to the birth canal. This condition, called vasa previa, necessitates a cesarean birth in order to prevent the rupturing of fetal blood vessels.
From the Latin “vasa” for vessels, “pre” for before or in front of, and “via” for road, vasa previa literally means “vessels in the way.”
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.
When vasa previa is left undiagnosed and the baby is delivered vaginally, there is as much as a 95% mortality rate for the fetus. When the amniotic sac pops and the baby passes through the cervix, the low-lying blood vessels inevitably rupture, causing severe bleeding. With so little blood in its body to begin with, such bleeding is usually fatal for the baby.
Ultrasounds are used to diagnose vasa previa. Throughout pregnancy, doctors use ultrasonography to look for abnormalities. If vasa previa is suspected, a doctor can perform a transvaginal ultrasound, in which a device is inserted into the vagina in order to see any blood vessels that may be crossing over the cervix.
If the vasa previa was not diagnosed, and the baby is delivered vaginally, doctors will soon notice that there is a problem. They will perform resuscitation and blood transfusions to counteract the baby’s blood loss.
When diagnosed, vasa previa is treated with careful monitoring of the mother and fetus as well as a planned c-section. Sometimes, the vasa previa resolves during the third trimester. The mother may have to go in for testing twice a week, go on pelvic rest, or in the late stages of pregnancy, be hospitalized.
If at any point the mother or fetus are in danger, an emergency c-section is performed. Since the baby will not be carried to full term, doctors prescribe steroids to help the baby grow as fast as possible while still in the womb.
“Vasa Previa: An Avoidable Obstetric Tragedy” by Sujata Datta et al., Journal of Obstetrics & Gynecology of India, 2015.
“Vasa previa: diagnosis and management” by Morgan Swank et al., American Journal of Obstetrics and Gynecology, 2016.
“Frequency of spontaneous resolution of vasa previa with advancing gestational age” by Rebecca Klahr et al., American Journal of Obstetrics and Gynecology, 2019.
“Vasa previa: time to make a difference” by Yinka Oyelese, American Journal of Obstetrics and Gynecology, 2019.