The placenta is attached to the inner wall of the uterus. It most often anchors to the top, side, front, or back of the uterus, and it connects to the baby through the umbilical cord. This dual attachment is what makes the placenta work: it sits between mother and baby, pulling oxygen and nutrients from maternal blood and delivering them to the fetus.
Where on the Uterus It Attaches
The placenta can implant almost anywhere on the uterine wall and still function normally. The most common positions are the top of the uterus (called the fundus), the front wall (anterior), the back wall (posterior), or either side. An anterior placenta sits between the baby and your abdomen, which sometimes muffles fetal kicks early on. A posterior placenta sits closer to your spine. Both are completely normal, and most people learn their placenta’s position during a mid-pregnancy ultrasound.
Problems arise when the placenta attaches too low in the uterus, near or over the cervix. If the placental edge sits within 2 centimeters of the cervical opening, it’s classified as a low-lying placenta. If it covers the opening entirely, that’s placenta previa. Early in pregnancy a low placenta often resolves on its own as the uterus grows and the placenta effectively “migrates” upward. When it doesn’t, a cesarean delivery is typically needed because the placenta would block the baby’s exit.
How the Placenta Anchors Itself
The placenta doesn’t just rest against the uterine wall. It actively invades it. In the first days after an embryo implants, specialized cells on the outer surface of the embryo begin burrowing into the uterine lining, which has transformed into a thick, blood-rich layer called the decidua. These invasive cells form a shell-like structure that firmly secures the developing placenta to the uterine wall.
The process gets more sophisticated from there. Some of these cells push deeper into the uterine tissue and target the small spiral-shaped arteries that supply blood to the area. They actually replace the cells lining those arteries, remodeling them from narrow vessels into wide, high-flow channels. This remodeling is what allows large volumes of maternal blood to wash through the placenta and exchange oxygen, nutrients, and waste with fetal blood, all without the two bloodstreams ever directly mixing.
Early in pregnancy, these cells also temporarily plug the spiral arteries, creating a low-oxygen environment around the embryo. That sounds counterintuitive, but it protects the fragile early placenta from oxygen damage and promotes healthy blood vessel development. By the end of the first trimester, the plugs dissolve and oxygen-rich maternal blood begins flowing freely into the placenta.
How the Placenta Connects to the Baby
On the fetal side, the placenta connects to the baby through the umbilical cord. The cord normally inserts near the center of the placenta, which is the most secure attachment point. It contains two arteries that carry oxygen-depleted blood from the baby to the placenta and one vein that returns freshly oxygenated blood back to the baby.
Sometimes the cord inserts off-center. In marginal cord insertion, it attaches within 2 centimeters of the placenta’s edge rather than the middle. This happens in a small percentage of pregnancies and is usually monitored but not dangerous on its own. A rarer variation called velamentous insertion occurs when the cord’s blood vessels travel unprotected across the membranes before reaching the placenta, which carries a higher risk of complications during delivery.
When the Placenta Attaches Too Deeply
In a normal pregnancy, the placenta grows into the uterine lining but not beyond it. In placenta accreta spectrum, the placenta burrows too deep. The mildest form involves attachment directly to the uterine muscle layer beneath the lining. In more severe cases, the placenta grows through the muscle or even penetrates the outer surface of the uterus entirely.
This condition has become significantly more common over recent decades. Studies from the 1970s and 1980s found it in roughly 1 in 2,500 to 1 in 4,000 pregnancies. A 2016 U.S. study found a rate of 1 in 272. The primary driver is the rising rate of cesarean deliveries, because scar tissue from a prior cesarean creates a spot where the placenta can invade too deeply. The risk climbs steeply with each additional cesarean: 0.3% after one prior cesarean, rising to nearly 7% after five or more.
Placenta previa combined with prior cesarean scars is an especially high-risk combination. For someone with placenta previa and no prior cesareans, the chance of accreta is about 3%. With three prior cesareans and previa, the risk jumps to 40%.
How the Placenta Detaches After Birth
After the baby is born, the uterus contracts with more force than at any point during labor. These powerful contractions shear the placenta off the uterine wall, compress the area where it was attached, and constrict blood vessels to limit bleeding. Three signs tell your care team the placenta has separated: the uterus changes shape and becomes more round and firm, a gush of blood appears, and the visible portion of the umbilical cord lengthens as the placenta drops down.
The entire process typically takes 5 to 30 minutes. If the placenta hasn’t delivered within 30 minutes (or 60 minutes when labor is managed without medication to speed contractions), it’s classified as a retained placenta. This happens when part or all of the placenta stays stuck to the uterine wall, and it requires intervention to prevent heavy bleeding.
Retained placenta is more likely in cases of undiagnosed accreta, where the placenta has grown too deeply to separate on its own. It can also happen when the cervix closes before the placenta passes through, or when a small fragment breaks off and stays behind while the rest delivers normally.