Velamentous cord insertion (VCI) is an abnormal pregnancy condition where the umbilical cord attaches to the fetal membranes instead of directly into the center of the placenta. Normally, the cord inserts directly into the placental mass, providing a secure connection for nutrient and oxygen exchange. VCI is relatively rare, occurring in about 1% to 1.5% of singleton pregnancies, but is more common in multiple gestations. This atypical attachment leaves the blood vessels connecting the fetus to the placenta without their natural cushioning, creating a vulnerability that requires careful medical management.
The Anatomy of Velamentous Insertion
In a healthy pregnancy, the umbilical cord inserts centrally or slightly off-center into the placenta. The vessels within the cord are encased and protected by Wharton’s jelly, a thick, gelatinous substance. This protective sheath shields the blood vessels from external pressure, kinking, or rupture, ensuring an uninterrupted flow of blood and nutrients to the fetus.
With velamentous insertion, the umbilical cord ends prematurely in the amniotic membranes outside the placental disk. The umbilical vessels then separate and traverse across the membranes—the thin tissue lining the inner wall of the uterus—before finally reaching the placenta’s surface. Lacking the cushion of Wharton’s jelly, this unprotected segment of vessels is highly susceptible to compression or injury within the amniotic sac.
The vessels are exposed and run free across the membranes, resembling threads connecting the cord stump to the placental tissue. This structural anomaly means the vessels travel a less direct and exposed route. This can increase resistance to blood flow and makes the system less mechanically resilient, leaving the vessels vulnerable to any force or movement inside the uterus.
How Velamentous Insertion is Detected
VCI is typically asymptomatic for the mother, relying entirely on prenatal imaging technology for detection. The condition is most often identified during the routine second-trimester anatomy scan, usually taking place between 18 and 22 weeks of gestation. During this detailed ultrasound, the sonographer or physician systematically visualizes the exact point where the umbilical cord enters the placenta.
Color Doppler ultrasound is instrumental in confirming the diagnosis. This technology highlights blood flow, allowing the practitioner to trace the path of the umbilical vessels to their insertion point. When VCI is present, Doppler imaging clearly shows the vessels diverging and running through the membranes before they coalesce into the placenta, rather than inserting directly into the placental mass.
Although the sensitivity of ultrasound for detecting abnormal cord insertions varies, it has high specificity, meaning a positive finding is likely accurate. Early detection allows the medical team to proactively monitor the pregnancy and plan for a safer delivery, especially if exposed vessels are found near the cervix. Systematic assessment of the cord insertion site is a standard part of mid-pregnancy ultrasound screening.
Potential Complications for Mother and Baby
The primary risks associated with VCI stem from the exposed and vulnerable nature of the vessels running across the membranes. During pregnancy, the condition is linked to an increased risk of Fetal Growth Restriction (FGR). This restricted growth is believed to be caused by less efficient blood flow and increased resistance in the elongated, unprotected vessels delivering nutrients to the fetus.
VCI is also associated with a higher likelihood of preterm birth. The most severe complications occur when the membranes rupture, either during labor or spontaneously before labor begins. The exposed vessels are at significant risk of compression, kinking, or tearing, which can lead to acute fetal blood loss and emergency situations.
The most dangerous potential outcome is Vasa Previa, occurring in approximately 6% of VCI cases. Vasa previa is diagnosed when the exposed vessels cross directly over the internal cervical opening (the os), lying between the fetus and the birth canal. If the amniotic sac breaks, these vessels can tear, resulting in rapid, life-threatening hemorrhage for the fetus. This complication carries a high risk of perinatal mortality if not managed before the onset of labor.
Monitoring and Delivery Planning
Once VCI is diagnosed, increased antenatal surveillance is implemented to mitigate potential risks. Monitoring often includes more frequent ultrasound assessments, typically every four to six weeks starting around 24 weeks of gestation, to track fetal growth and amniotic fluid levels. Doctors may also use non-stress tests and Biophysical Profiles later in the third trimester to monitor the baby’s well-being.
The most important management decision involves determining the presence of vasa previa. If unprotected vessels are found close to or covering the cervix, a vasa previa diagnosis is made, dramatically changing the delivery plan. In this scenario, a planned Cesarean section is almost always scheduled before the onset of labor, often between 34 and 37 weeks, to prevent catastrophic vessel rupture.
For cases of isolated VCI, where vasa previa is ruled out, a vaginal delivery may still be possible, but labor is managed with extreme caution. Medical teams remain highly vigilant for any signs of fetal distress, such as changes in the fetal heart rate pattern. The goal of proactive monitoring is to ensure the baby is delivered safely before any compromise to the fragile, unprotected umbilical vessels occurs.