Placenta: How it works, what's normal

The placenta plays a crucial role during pregnancy. Find out what the placenta does, issues that might affect the placenta and how the placenta is delivered.

By Mayo Clinic Staff

If you're pregnant, you might wonder what the placenta does and what factors can affect the placenta. Get the facts about this important fetal organ.

What does the placenta do?

The placenta is an organ that develops in your uterus during pregnancy. This structure provides oxygen and nutrients to your growing baby and removes waste products from your baby's blood. The placenta attaches to the wall of your uterus, and your baby's umbilical cord arises from it. In most pregnancies, the placenta attaches at the top or side of the uterus.

What affects placental health?

Various factors can affect the health of the placenta during pregnancy, some modifiable and some not. For example:

  • Maternal age. Certain placental problems are more common in older women, especially after age 40.
  • Premature rupture of the membranes. During pregnancy, your baby is surrounded and cushioned by a fluid-filled membrane called the amniotic sac. If the sac leaks or breaks before labor begins, the risk of certain placental problems increases.
  • High blood pressure. High blood pressure can affect your placenta.
  • Twin or other multiple pregnancy. If you're pregnant with more than one baby, you might be at increased risk of certain placental problems.
  • Blood-clotting disorders. Any condition that either impairs your blood's ability to clot or increases its likelihood of clotting increases the risk of certain placental problems.
  • Previous uterine surgery. If you've had a previous surgery on your uterus, such as a C-section or surgery to remove fibroids, you're at increased risk of certain placental problems.
  • Previous placental problems. If you've had a placental problem during a previous pregnancy, you might be at increased risk of experiencing it again.
  • Substance abuse. Certain placental problems are more common in women who smoke or use illegal drugs, such as cocaine, during pregnancy.
  • Abdominal trauma. Trauma to your abdomen — such as from a fall or other type of blow to the abdomen — increases the risk of certain placental problems.

What are the most common placental problems?

During pregnancy, the most common placental problems include placental abruption, placenta previa and placenta accreta. These conditions can cause potentially heavy vaginal bleeding. After delivery, retained placenta is also sometimes a concern.

  • Placental abruption (abruptio placentae). If the placenta peels away from the inner wall of the uterus before delivery — either partially or completely — it's known as placental abruption. Placental abruption can cause varying degrees of vaginal bleeding and pain or cramping. It might also deprive the baby of oxygen and nutrients. In some cases, early delivery is needed.
  • Placenta previa. This condition occurs when the placenta partially or totally covers the cervix — the outlet for the uterus. Placenta previa is more common early in pregnancy and might resolve as the uterus grows. Placenta previa can cause severe vaginal bleeding before or during delivery. A C-section delivery usually is required if the placenta previa is present at the time of delivery.
  • Placenta accreta. This condition occurs when the blood vessels of the placenta grow too deeply into the uterine wall. Placenta accreta can cause vaginal bleeding during the third trimester of pregnancy and severe blood loss after delivery. Treatment might require a C-section delivery followed by surgical removal of the uterus (abdominal hysterectomy). More-aggressive forms of this problem can also occur if the placenta invades the muscles of the uterus (placenta increta) or if the placenta grows through the uterine wall (placenta percreta).
  • Retained placenta. If the placenta isn't delivered within 30 to 60 minutes after childbirth, it's known as retained placenta. Retained placenta might occur because the placenta becomes trapped behind a partially closed cervix or because the placenta is still attached to the uterine wall — either loosely (adherent placenta) or deeply (placenta accreta). Left untreated, a retained placenta can cause severe infection or life-threatening blood loss in the mother.
March 13, 2015 See more In-depth