The umbilical cord is a vital connection during pregnancy, acting as a lifeline between the developing baby and the mother. It ensures the transfer of nutrients and oxygen and the removal of waste products. In pregnancies involving twins, these lifelines introduce specific considerations that can influence the development and well-being of both babies.
Umbilical Cord Basics
The umbilical cord is a flexible, tube-like structure that typically contains three blood vessels. One umbilical vein carries oxygen-rich blood and vital nutrients from the placenta to the developing fetus. Two umbilical arteries transport deoxygenated blood and waste products away from the fetus, back to the placenta. This continuous two-way exchange is fundamental for fetal development.
Wharton’s jelly, a protective, gelatinous substance, encases these vessels. It provides structural support and prevents compression, ensuring uninterrupted blood flow. The cord generally attaches to the center of the placenta, facilitating efficient nutrient delivery and waste removal.
How Twin Pregnancies Affect Umbilical Cord Structure
Twin pregnancies present varying umbilical cord structures depending on how the twins developed within the womb. In dichorionic-diamniotic (DiDi) twin pregnancies, each twin has its own separate placenta and amniotic sac. Each baby develops its own distinct umbilical cord, functioning independently.
For monochorionic-diamniotic (MoDi) twins, both babies share a single placenta but have separate amniotic sacs. Each twin typically has its own umbilical cord originating from this shared placenta. The arrangement of blood vessels within the shared placenta can be intricate.
The most complex scenario arises with monochorionic-monoamniotic (MoMo) twins, where both babies share a single placenta and a single amniotic sac. The umbilical cords of both twins float freely within the same confined space. This close proximity increases the possibility of entanglement.
Variations in how the umbilical cord inserts into the placenta can also impact twin pregnancies.
A marginal cord insertion occurs when the cord attaches at the edge of the placenta. This insertion type is more common in twin pregnancies and can affect nutrient transfer.
Velamentous cord insertion occurs when the umbilical vessels separate and travel unprotected through the fetal membranes before reaching the placental tissue. This leaves the vessels vulnerable to compression or damage and is more frequent in twin pregnancies, especially monochorionic types.
Specific Challenges of Shared Cords
The shared placental or amniotic sac environment in some twin pregnancies can lead to unique complications.
Twin-to-Twin Transfusion Syndrome (TTTS) is a condition occurring in identical twins who share a placenta, where uneven blood vessel connections cause an imbalanced blood flow. One twin, the “donor,” gives away more blood than it receives, leading to lower blood volume and less amniotic fluid, while the “recipient” twin receives too much, resulting in excess blood volume and amniotic fluid.
Selective Fetal Growth Restriction (sFGR) occurs in monochorionic twins due to unequal placental sharing. One twin receives fewer nutrients and grows significantly smaller than the other.
Twin Anemia Polycythemia Sequence (TAPS) involves an unequal red blood cell distribution between monochorionic twins. This occurs through small connections in the shared placenta, leading to one twin becoming anemic (having too few red blood cells) and the other becoming polycythemic (having too many). Unlike TTTS, TAPS typically does not involve significant differences in amniotic fluid levels.
Twin Reversed Arterial Perfusion (TRAP) sequence is a rare condition where one twin, known as the “pump twin,” supplies blood to an abnormally developed “acardiac” twin that lacks a functioning heart and often an upper body. The pump twin’s heart must work harder to circulate blood for both, placing it at risk for heart failure.
In monochorionic-monoamniotic pregnancies, cord entanglement is a concern due to the absence of a separating membrane. The twins’ umbilical cords can intertwine, potentially restricting blood flow to one or both babies. While common in MoMo pregnancies, the severity and impact on blood flow vary.
Monitoring and Management of Twin Cords
Given the potential complexities, twin pregnancies, especially those sharing a placenta, undergo careful monitoring.
Regular ultrasound scans are a primary tool used to track the growth of each fetus, assess amniotic fluid levels, and examine the structure and blood flow within the umbilical cords. These frequent scans help detect early signs of potential complications.
Doppler studies are often performed as part of these ultrasounds to specifically measure blood flow within the umbilical cord vessels and fetal brains. This specialized imaging can identify abnormal flow patterns that might indicate unequal sharing of blood or other circulatory issues between the twins. Such detailed assessments are crucial for timely intervention.
Complex twin pregnancies often require the expertise of maternal-fetal medicine specialists, who are obstetricians with advanced training in high-risk pregnancies. These specialists can develop tailored management plans and provide specialized care throughout the pregnancy. In certain severe cases of complications like TTTS, fetal interventions such as laser surgery may be considered to correct the imbalanced blood flow in the placenta.