Vasa previa is a rare pregnancy condition in which fetal blood vessels run across or near the opening of the cervix, unprotected by the umbilical cord or placenta. Because these vessels sit directly in the path the baby would take during delivery, they can tear when the membranes rupture (water breaking), causing the baby to lose blood rapidly. When diagnosed before labor, survival rates reach 97%. When it goes undetected, that number drops to roughly 44%.
How Vasa Previa Develops
Normally, the blood vessels connecting the baby to the placenta are bundled safely inside the umbilical cord, which inserts directly into the placental tissue. In vasa previa, some of those vessels travel through the fetal membranes without that protective covering, and they happen to cross over the internal opening of the cervix.
There are two types. Type 1, the more common form, happens when the umbilical cord inserts into the membranes instead of directly into the placenta. This is called a velamentous cord insertion. The exposed vessels then have to travel across the membranes to reach the placental edge, and if that path crosses the cervix, it becomes vasa previa. Type 2 occurs when the placenta has an extra lobe (a succenturiate lobe) or is split into two lobes, and the vessels connecting the lobes run across the cervix.
In either case, the core problem is the same: fragile fetal blood vessels sit in a location where they’re vulnerable to compression or rupture, especially once labor starts or the membranes break.
Who Is at Higher Risk
Vasa previa is uncommon in the general population, but certain factors raise the odds significantly. Pregnancies conceived through in vitro fertilization (IVF) carry a notably higher risk, with an estimated incidence of about 1 in 260 IVF pregnancies. The reasons aren’t entirely clear, but the association is strong enough that clinicians often screen IVF pregnancies more closely.
Other risk factors include:
- Low-lying placenta or placenta previa: When the placenta sits in the lower part of the uterus, fetal vessels are more likely to cross near the cervix.
- Multilobed placenta: A placenta with a succenturiate (accessory) lobe creates connecting vessels that may pass over the cervix.
- Velamentous cord insertion: An umbilical cord that inserts into the membranes rather than directly into the placenta.
- Multiple pregnancies: Carrying twins or more increases the chance of abnormal placental and cord anatomy.
- Advanced maternal age and prior cesarean deliveries: Both are associated with rising rates of placental abnormalities, including vasa previa.
In fact, most cases that are caught before delivery are found incidentally when a woman undergoes a transvaginal ultrasound to evaluate a low-lying placenta or placenta previa.
Why It’s Dangerous
A baby’s total blood volume is small. A full-term newborn has roughly 80 milliliters of blood per kilogram of body weight, so even a modest amount of bleeding can be catastrophic. When membranes rupture and a fetal vessel tears, the baby can lose a life-threatening volume of blood within minutes. The bleeding is painless for the mother, which makes it deceptive. Before routine ultrasound existed, the classic presentation was a triad: ruptured membranes, painless vaginal bleeding, and sudden fetal distress or death.
Even without rupture, the vessels can be compressed by the baby’s head pressing against the cervix during labor, cutting off blood flow. This is why vasa previa is dangerous throughout the late stages of pregnancy and labor, not only at the moment the water breaks.
How It’s Detected
Prenatal ultrasound, particularly transvaginal ultrasound with color Doppler imaging, is the primary tool for identifying vasa previa. Color Doppler highlights blood flow, making it possible to see fetal vessels crossing near the cervix. The first prenatal diagnosis by ultrasound was reported in 1987, and the technology has improved substantially since then.
Current screening approaches typically involve assessing where the umbilical cord inserts into the placenta during the second-trimester anatomy scan (usually around 18 to 22 weeks). If the cord insertion looks abnormal or the placenta is low-lying, a follow-up transvaginal ultrasound with Doppler is performed. In women with known risk factors, such as IVF conception or a succenturiate lobe, clinicians may look for vasa previa more deliberately. A third-trimester confirmation scan is then used to verify whether the condition persists, since some low-lying placentas migrate upward as the uterus grows.
What Happens After Diagnosis
Once vasa previa is confirmed, the pregnancy is managed with the goal of delivering the baby by planned cesarean section before labor begins or the membranes rupture on their own. The timing of that delivery balances two competing risks: the danger of an emergency vessel rupture if the pregnancy continues too long, and the complications of prematurity if the baby is delivered too early.
The Society for Maternal-Fetal Medicine and other organizations recommend considering hospital admission around 30 weeks of gestation for close monitoring. Being in the hospital means that if bleeding, contractions, or membrane rupture occur, an emergency cesarean can happen within minutes rather than the time it would take to travel to a hospital. About 28% of vasa previa cases require an emergency preterm delivery, which is why inpatient monitoring is often recommended despite the inconvenience of a long hospital stay.
Steroid injections are typically given between 28 and 32 weeks to accelerate the baby’s lung development, preparing for the possibility of an early delivery. The planned cesarean is generally scheduled in the late preterm period, often around 34 to 37 weeks, depending on the clinical situation and whether there have been any bleeding episodes or signs of labor.
Prenatal Diagnosis Changes Everything
The difference between a diagnosed and undiagnosed case of vasa previa is stark. A landmark study of 155 pregnancies with vasa previa found that 97% of babies survived when the condition was identified before birth, compared to just 44% when it was discovered only after membranes ruptured during labor. That gap exists because prenatal diagnosis allows a controlled cesarean delivery before the vessels are ever at risk of tearing.
The challenge is that vasa previa doesn’t cause symptoms until something goes wrong. There’s no pain, no warning contractions, no obvious sign that something is different about the pregnancy. Vaginal bleeding in the second or third trimester can be a clue, and any unexplained bleeding should prompt an evaluation, but many cases are entirely silent until labor. This is why ultrasound screening, especially in women with risk factors, is so important. The condition itself is dangerous, but it’s also highly treatable when caught in time.