What Are Placentas and How Do They Support Pregnancy?

The placenta is a temporary organ that develops in the uterus during pregnancy, acting as the interface between the mother and the developing baby. It attaches to the uterine wall and connects to the fetus via the umbilical cord. Its primary purpose is to facilitate the transfer of everything the fetus needs to grow while removing what it does not.

The Making of the Placenta: Development and Structure

The formation of the placenta begins shortly after a fertilized egg implants into the uterine wall. The process is initiated by specialized cells called trophoblasts, which make up the outer layer of the early-stage embryo. These cells multiply and invade the uterine lining, establishing the foundational connections that will become the mature placenta. This invasion process is carefully controlled, allowing the organ to anchor itself securely.

As pregnancy progresses, the placenta develops into a disc-shaped organ composed of tissues from both the mother and the fetus. The fetal side, known as the chorionic plate, is smooth and gives rise to the umbilical cord. From this plate, tree-like structures called chorionic villi branch out, containing a dense network of fetal blood vessels that are the primary sites for exchange.

The maternal side, the basal plate, is a rougher surface attached to the uterine wall. This side is bathed in maternal blood that flows into the intervillous space, allowing for close proximity between maternal and fetal circulation without the blood mixing. When fully developed, the placenta measures about 10 inches in length and weighs around one pound.

Vital Functions of the Placenta During Pregnancy

The placenta’s primary function is facilitating a two-way exchange. It transfers oxygen and nutrients like glucose, amino acids, and vitamins from the mother’s bloodstream to the fetus. Simultaneously, it removes carbon dioxide, urea, and other metabolic wastes from the fetal bloodstream, transporting them into the mother’s circulation for disposal by her organs.

The placenta also functions as an endocrine organ, producing several hormones that regulate pregnancy. It secretes human chorionic gonadotropin (hCG), the hormone detected in pregnancy tests, which stimulates the ovaries to produce progesterone in early pregnancy. As the placenta matures, it manufactures its own progesterone and estrogen to maintain the uterine lining and support fetal growth. Another hormone, human placental lactogen (hPL), helps prepare the mother’s body for breastfeeding.

Finally, the placenta acts as a protective shield. It forms a barrier that prevents many harmful microorganisms and toxins from reaching the fetus. It also helps to hide the fetus from the mother’s immune system, preventing it from being rejected. Toward the end of pregnancy, the placenta transfers antibodies from the mother to the fetus, providing passive immunity that protects the newborn during its first few months of life.

The Placenta’s Journey After Birth

Once the baby is delivered, the placenta’s role is complete. The delivery of the placenta marks the third stage of labor, which occurs within 30 minutes of the baby’s birth. The uterus continues to contract, causing the placenta to detach from the uterine wall and be expelled through the birth canal as the “afterbirth.”

After delivery, healthcare providers examine the placenta to ensure it is whole and that no fragments remain in the uterus. Retained placental tissue can lead to serious complications for the mother, such as infection or postpartum hemorrhage. The inspection also confirms the organ’s general health and that the detachment was clean.

The disposal of the placenta varies based on hospital protocols and personal or cultural preferences. In most medical settings, the placenta is treated as medical waste and incinerated. However, some parents choose to take their placenta home for cultural traditions, such as burying it in a meaningful location or consuming it in a practice known as placentophagy.

Common Placental Complications

Although most pregnancies proceed without issue, several placental conditions can affect the health of both the mother and fetus. These complications require careful medical monitoring, and early detection through routine prenatal care is important for management.

Common placental complications include:

• Placenta previa: The placenta attaches low in the uterus, covering the cervix, which can cause severe bleeding and may require a cesarean section.
• Placental abruption: The placenta prematurely separates from the uterine wall, depriving the fetus of oxygen and causing significant maternal bleeding.
• Placenta accreta: The placenta attaches too deeply into the uterine wall, making detachment difficult and potentially causing massive hemorrhage.
• Placental insufficiency: The placenta cannot deliver adequate oxygen and nutrients, which can result in poor fetal growth, known as intrauterine growth restriction (IUGR).