Placenta 101: What Every Expectant Mother Should Know

Content medically reviewed by Rachel Marohl, MD

You may have heard the word “placenta” and know that it has to do with pregnancy. But until you are expecting a baby, you might not know what it is. 

Not long after conception, the placenta develops within the womb and attaches to the uterine wall, and it continues to grow during pregnancy. The umbilical cord connects the growing baby to the placenta, which performs many vital functions.

“The placenta is an essential organ that is critical for the developing fetus,” said Dr. Rachel Marohl, OB/GYN at Essentia Health. “With the help of the umbilical cord, the placenta helps keep your baby healthy throughout your pregnancy. Knowing how the placenta forms and what it does can help you understand what is happening within your body and how your baby receives everything it needs to grow and stay healthy.”

Understanding How the Placenta Develops

After an egg is fertilized and travels down a fallopian tube to the uterus, the cells of the fertilized egg, or zygote, divide and form a cluster of cells called a blastocyst. The blastocyst then implants in the uterine wall. After implantation, which happens around the tenth day after fertilization, the blastocyst’s inner cells begin to form the embryo, while the outer cells begin to develop into the placenta.

The placenta forms little by little over the first three months. Then, by the end of the fourth month of pregnancy, it grows parallel to the uterus. When the placenta is fully developed, it is a spongy disk about eight inches in diameter that is attached to the uterus.

At the same time, some placental cells form a layer of membranes around the outside of the blastocyst, while others create an inner layer, or the amniotic sac. At this point, the blastocyst is now an embryo. A clear liquid called amniotic fluid fills the sac, and the embryo floats in this fluid. The embryo then begins developing internal organs and external body parts. 

The heart and major blood vessels are the first to form, at just more than two weeks after fertilization. At about three weeks, the heart pumps fluid and blood through the blood vessels. Other organs begin to develop at this time as well. Almost all are fully formed by 12 weeks of pregnancy, with the exception of the brain and spinal cord, which continue to develop throughout the remainder of the pregnancy. 

What Does the Placenta Do for Your Baby?

The placenta performs many essential jobs to keep your baby alive and healthy. In essence, the placenta does the work of the baby’s lungs, liver, kidneys, and gastrointestinal tract while the baby is in the womb.

Some of the placenta’s functions include –

• Carrying vital nutrients and oxygen from the mother to the fetus
• Producing hormones, such as human chorionic gonadotropin (HCG), which encourages the ovaries to produce estrogen and progesterone to support the pregnancy and stops the ovaries from releasing more eggs
• Removing carbon dioxide and other waste products from the fetus and directing them to the mother

During the development of the placenta, tiny structures that resemble hairs, called villi, burrow into the uterine wall. Villi, which spread out like tree branches, are part of the embryo’s circulatory system. Blood vessels run through the villi, while the mother’s blood vessels travel next to the villi, filling the space with her blood. Nutrients and oxygen from the maternal blood pass through a thin membrane to the embryo.

Rare Problems of the Placenta

“A healthy placenta is essential for a healthy baby,” Dr. Marohl said. “This means that problems with the placenta must be treated promptly.”

Rarely, potentially serious complications involving the placenta can occur. One of these is called placenta previa, a condition in which the placenta remains in the lower part of the uterus, covering the opening to the cervix. 

While it is common for the placenta to grow low in the uterus during early pregnancy, it usually moves up as the uterus stretches, leaving the opening to the cervix clear for the delivery of the baby. But if the placenta stays low and remains near the opening of the cervix, this is called a low-lying placenta. If it covers all or part of the cervix, this is placenta previa, which can cause severe bleeding and put you and your baby at risk.

“If you experience sudden, severe bleeding around the end of the second trimester or beginning of the third trimester, seek medical care immediately,” Dr. Marohl said. “You may need bed rest, and your provider may recommend delivering your baby early to keep both you and the baby safe.”

Another problem that can occur is called placental abruption, which means the placenta separates from the uterine lining before delivery. Although rare, placental abruption is most common in the last few weeks of pregnancy and requires emergency care. Possible symptoms include abdominal or back pain, uterine contractions, and vaginal bleeding.

What Happens to the Placenta After Delivery?

Shortly after the birth of your baby, you will deliver the placenta. Your uterus will continue to contract mildly and the placenta will detach, traveling down the birth canal to be delivered vaginally. If you deliver your baby via a C-section, your provider will remove the placenta as part of the procedure.

Your provider will check the placenta to make sure no pieces have been left behind. If any part of the placenta remains, it needs to be removed promptly to prevent infection and bleeding. 

“When the placenta is delivered, this is frequently referred to as afterbirth,” Dr. Marohl said. “This is the final stage of labor and is an important part of the process, as the placenta needs to be completely expelled to prevent any complications. At this point, the placenta has fulfilled its purpose of keeping your baby healthy.”