The umbilical cord is the biological lifeline connecting a developing fetus to the placenta, acting as a tube for life support. This flexible structure contains one large vein that delivers oxygen and nutrients from the placenta to the fetus and two smaller arteries that carry deoxygenated blood and waste products back. Because the cord is the sole supply line for a developing baby, the simple answer to how many umbilical cords twins have is always two, one for each twin. The complexity arises not from the cord count itself, but from the number of placentas and amniotic sacs involved, which directly impacts the pregnancy’s risk profile.
Cord Count for Fraternal Twins
Fraternal twins, also known as dizygotic twins, originate from a distinct process where two separate eggs are released and fertilized by two different sperm during the same cycle. This means they are genetically no more alike than any other siblings born at different times, and they can be the same sex or different sexes.
This separate fertilization leads to two completely independent implantations within the uterus. Each fertilized egg develops its own placenta, known as a dichorionic structure, and its own surrounding amniotic sac, termed diamniotic.
Consequently, each twin has a dedicated umbilical cord connecting it to its own, separate placenta. This arrangement represents the least complicated twin scenario in terms of fetal support structures, as there is no sharing of blood supply or amniotic space.
Cord Count for Identical Twins
Identical twins result from a single egg fertilized by a single sperm, with the resulting zygote spontaneously splitting into two embryos early in development. The timing of this split determines the structural arrangement, including the number of placentas and sacs, but regardless of the timing, there are always two distinct umbilical cords, one for each baby.
If the split occurs very early, within the first three days after fertilization, the twins will develop like fraternal twins, with two separate placentas and two separate amniotic sacs (dichorionic-diamniotic). The most common scenario occurs when the split happens between four and eight days, resulting in a shared placenta but two separate amniotic sacs (monochorionic-diamniotic). In the rarest instance, a split after eight days leads to the twins sharing both a single placenta and a single amniotic sac (monochorionic-monoamniotic).
Significance of Placental and Cord Arrangement
The arrangement of the placenta and sacs, specifically in identical twin pregnancies, dictates the severity of potential complications and the required level of medical monitoring. When twins share a placenta (monochorionic), the blood vessels within that placenta often connect, which can lead to an unbalanced transfer of blood flow between the babies.
Twin-to-Twin Transfusion Syndrome (TTTS)
This unequal sharing can result in Twin-to-Twin Transfusion Syndrome (TTTS), which affects 10–15% of monochorionic twins. In TTTS, one twin, the donor, receives too little blood and can become anemic and have low amniotic fluid, while the other, the recipient, receives too much blood, straining the heart and causing excess amniotic fluid.
Cord Entanglement
The highest risk arrangement is monochorionic-monoamniotic, where the absence of a dividing membrane allows the two separate umbilical cords to float freely within the same sac. This free movement creates a significant risk of cord entanglement, where the two cords twist around each other, potentially restricting or cutting off the blood supply to one or both fetuses. Due to these unique and serious risks, monochorionic twin pregnancies require intensive fetal surveillance, often involving frequent ultrasounds and non-stress tests, to detect complications early.