The umbilical cord acts as the lifeline for a developing fetus, connecting it to the placenta to facilitate the exchange of gases, nutrients, and waste products. When a single pregnancy occurs, a single cord links the fetus to one placenta, but the arrangement in a twin pregnancy is far more complex. Whether twins have separate umbilical cords depends entirely on how the initial fertilization and cell division events occurred, establishing the specific structural setup inside the uterus.
Fraternal vs. Identical Twins: Defining the Origin
The fundamental difference between twin types dictates the entire placental and cord structure. Fraternal twins (dizygotic twins) result from the fertilization of two separate eggs by two separate sperm. Since they originate from two distinct zygotes, they share approximately 50% of their DNA, similar to any other siblings.
Identical twins (monozygotic twins) begin with a single fertilized egg that splits into two embryos early in development. Because they share the same genetic material, they are always the same sex and share 100% of their DNA. The timing of this split determines the subsequent structures that support the twins, including the placenta and surrounding membranes.
Dichorionic Twins: The Fully Separate System
The most straightforward scenario for cord separation occurs in dichorionic diamniotic (Di/Di) pregnancies. Dichorionic means there are two separate chorions (outer membranes), giving each twin its own distinct placenta. Diamniotic indicates that each twin is also enclosed within its own separate amniotic sac.
All fraternal twins are inherently Di/Di because they start as two separate fertilized eggs. Approximately one-third of identical twins also develop this Di/Di structure if the single fertilized egg splits within the first three days after conception.
In this arrangement, each twin has its own umbilical cord connecting to its own independent placenta. The two placentas may implant separately or fuse together if they are close, sometimes appearing as a single mass on an ultrasound. Despite any fusion, the blood supplies remain functionally separate, ensuring each twin is nourished through its own dedicated cord and placental unit.
Monochorionic Twins: Shared Placenta and Cord Arrangement
When the single fertilized egg splits between the fourth and eighth day, the resulting identical twins share a single placenta, creating a monochorionic pregnancy. This arrangement occurs in about 70% of identical twin pregnancies, meaning the umbilical cords must insert into the same organ. The medical implications differ based on whether the twins also share the amniotic sac.
Monochorionic Diamniotic (Mo/Di)
Mo/Di twins share one placenta but are separated by individual amniotic sacs. Each fetus has its own umbilical cord, but both cords insert into the same single placenta. The primary concern is the vascular connections, called anastomoses, that form between the twins’ blood vessels beneath the surface of the shared placenta.
These connections can lead to unequal blood flow distribution, resulting in Twin-to-Twin Transfusion Syndrome (TTTS). In TTTS, one twin transfuses blood to the recipient twin through the shared connections, causing severe imbalances. Although the cords are technically separate, their shared destination creates a unique risk profile.
Monochorionic Monoamniotic (Mo/Mo)
The most extreme scenario is Mo/Mo twins, which occurs if the embryo splits after eight days. This rare type of pregnancy, accounting for less than one percent of all twins, involves the twins sharing both the single placenta and the single amniotic sac. Because there is no dividing membrane, the twins float freely in the same space.
The absence of a membrane allows the two cords to intertwine as the fetuses move, leading to a high risk of cord entanglement. Entanglement can cause the cords to knot or compress, restricting the flow of oxygen and nutrients. Due to the significant risk of cord compression, Mo/Mo pregnancies require intensive monitoring and often a planned early delivery.