Yes, the umbilical cord is directly connected to the placenta. It typically attaches to the center of the placental surface and serves as the sole lifeline between a developing baby and the mother’s blood supply. The cord contains blood vessels that carry oxygen and nutrients from the placenta to the fetus, and return waste products back to the placenta for the mother’s body to dispose of.
How the Cord Attaches to the Placenta
In most pregnancies, the umbilical cord inserts right into the middle of the placenta’s fetal-facing surface. The blood vessels inside the cord fan out across the placenta once they reach the insertion point, branching into smaller and smaller vessels that extend into finger-like projections of tissue. These tiny projections sit in pools of the mother’s blood, and that’s where the actual exchange happens: oxygen and nutrients pass from the mother’s blood into the baby’s blood vessels, while carbon dioxide and waste products move in the opposite direction. The mother’s blood and the baby’s blood never actually mix directly.
The cord itself is about 50 to 60 centimeters long at full term (roughly two feet) and about 12 millimeters in diameter. It contains two arteries and one vein. The two arteries carry used, oxygen-depleted blood from the baby to the placenta, while the single vein carries fresh, oxygen-rich blood and nutrients from the placenta back to the baby. This is the reverse of how arteries and veins work elsewhere in the body, which sometimes surprises people.
What Protects the Blood Vessels Inside
The three blood vessels inside the cord aren’t just floating loosely. They’re surrounded by a thick, jelly-like substance that cushions and protects them from being pinched or compressed as the baby moves around. This material has a texture similar to soft cartilage and is made up of proteins and sugars that give it a slippery, shock-absorbing quality. It’s one of the reasons the cord can twist, loop, and coil throughout pregnancy without cutting off blood flow in most cases.
When the Connection Forms
The cord-to-placenta connection starts taking shape remarkably early. Around the third week of pregnancy, the very first blood vessel precursors begin forming in the tissue that will become the umbilical cord. By the end of that third week, a basic network of tiny blood vessels has formed within the connecting stalk between the embryo and what will become the placenta.
Early in the fourth week, two arteries branch off from the embryo’s developing circulatory system and link up with this vascular network. Once the fetal heart starts pumping, around week four, blood begins flowing through the cord for the first time. By week seven, the umbilical cord is fully formed as a distinct structure, complete with its arteries, vein, and protective jelly coating, all wrapped in a membrane.
Variations in Where the Cord Attaches
Not every cord attaches neatly to the center of the placenta. In about 7% of placentas, the cord connects at the edge rather than the middle. This is called a marginal insertion, and while it’s usually monitored, it doesn’t always cause problems.
A less common variation, occurring in roughly 1.5% of singleton pregnancies, is when the cord doesn’t attach to the placenta itself at all. Instead, it inserts into the membranes surrounding the placenta, and the blood vessels travel unprotected across the membranes before reaching the placental tissue. This is called velamentous insertion, and it carries higher risk because those exposed vessels lack the protective jelly that normally cushions them. Twin pregnancies see these variations more often: about 6% have velamentous insertion and nearly 11% have marginal insertion.
These attachment variations are typically identified on ultrasound during pregnancy, which allows care teams to plan accordingly for delivery.
What Happens to the Connection After Birth
The cord remains attached to the placenta throughout delivery. After the baby is born, the cord is clamped and cut, separating the newborn from the placenta. But the placenta itself is still inside the uterus at this point. The third stage of labor, the time between the baby’s delivery and the delivery of the placenta, involves continued uterine contractions that cause the placenta to peel away from the uterine wall and be expelled. The cord comes out still attached to the placenta. On the baby’s side, the small stump left behind after the cord is cut dries up and falls off within one to three weeks, leaving the navel.