Placenta

What is the placenta?

The placenta is a temporary organ that connects the fetus to your uterus during pregnancy. The umbilical cord connects the fetus to the placenta. Together, they make it possible for your body’s nutrients to support the growth of the fetus.

There’s nothing much you can do to affect the growth and health of the placenta. But learning about it can help you better understand pregnancy and how fascinating this organ is.

What is the purpose of the placenta?

The placenta is vital to fetal development. It acts as the fetus’s lungs, kidneys and liver until birth.

The placenta’s main functions include:

• Providing the fetus with oxygen and nutrients
• Removing harmful waste products and carbon dioxide from the fetus
• Producing hormones that maintain your pregnancy and help the fetus grow
• Passing immunity from you to the fetus

Transferring nutrients and filtering waste

Your blood passes through the placenta and provides oxygen, glucose (sugar) and nutrients to the fetus through the umbilical cord. The placenta can also filter out harmful waste and carbon dioxide from the fetus’s blood.

As you get closer to delivery, the placenta passes antibodies from your immune system to the fetus to jumpstart immunity. This immunity sticks with your baby for several months after birth.

The placenta allows the exchange of all these things between your bloodstream and that of the fetus without ever mixing them.

Placenta hormones

The placenta also acts as an endocrine organ. It makes several hormones to support the pregnancy. The hormones affect both you and the fetus.

Some of these hormones include:

• Estrogen: Towards the end of pregnancy, estrogen triggers your uterus to grow and eventually allows your uterus to contract. It also triggers mammary gland development for lactation.
• Progesterone: This hormone maintains the lining of your uterus so the placenta and fetus can grow.
• Human chorionic gonadotrophin (HCG): This is the first hormone the placenta releases. It’s what pregnancy tests measure. It triggers the corpus luteum to maintain progesterone production.
• Human placental lactogen (hPL): Scientists think this hormone triggers the growth of your mammary glands. It may also help regulate your metabolism during pregnancy.
• Placental growth hormone (PGH): This hormone helps control fetal growth.
• Relaxin: This hormone relaxes your ligaments and muscles to make them more flexible. It softens your cervix at the end of pregnancy.

Both progesterone and hPL stimulate your appetite. This is why most people are extra hungry during pregnancy.

When does the placenta form?

The placenta begins to form after a fertilized egg implants in your uterine lining (endometrium). It has the same genetic makeup as the fetus.

It continues to grow throughout your pregnancy to support the fetus. The placenta starts as a few cells and grows to be several inches long.

When does the placenta take over?

The placenta takes over hormone production by the end of the first trimester (12 weeks of pregnancy). Up until this time, the corpus luteum handles most of the hormone production. Many people’s first-trimester symptoms of nausea and fatigue go away once the placenta takes over in the second trimester.

How is the placenta delivered?

You deliver the placenta shortly after your baby is born (usually within 30 minutes). This is called the afterbirth or the third stage of labor.

If you’ve had a vaginal delivery, your uterus will continue to contract to get rid of the placenta. Your healthcare provider may push on your belly or ask you for one final push. If you had a C-section, your provider removes the placenta through the same incision your baby came through.

In rare cases, parts of the placenta stay in your uterus after delivery. This can cause excessive bleeding (hemorrhage) and/or infection.

What happens to the placenta after birth?

Your provider will look at the placenta to check that it detached completely from your uterus. They’ll look for any signs of missing pieces or breakage.

After this, your provider will throw away the placenta after birth unless you choose otherwise.

You may choose to bank placental tissue through a stem cell bank. Placental tissue banking is when tissue and blood from the placenta are collected and stored after delivery. The placental tissues are rich in stem cells that can help treat certain conditions.

Sometimes, your provider might ask a pathologist to examine the placenta if:

• You or your baby has a fever or illness
• Your baby is born premature
• If your baby is small for their gestational age

What are the parts of the placenta?

You can think of the placenta as having three main parts:

1. The surface that faces the fetus, or the side the umbilical cord attaches to
2. The surface that’s attached to your uterine wall
3. The cavity between these two surfaces

The placenta contains mostly blood vessels contained within structures called villi. The placental cavity contains 30 to 40 of these villi.

You can think of the villi as trees. The trunk of the tree connects to the fetal surface of the placenta. Each “tree” has several branches that form a small lobe. The center of each lobe is over the opening of a spiral artery from the uterus side of the placenta.

Your blood then “bathes” the villi. This is how the fetus gets oxygen and nutrients. This is also how your body filters out waste from the fetus.

What does a placenta look like?

The placenta looks like a big disc of bumpy tissue rich in blood vessels. It has a gel-like consistency or is like a very wet sponge.

After delivery of the placenta, the side attached to your uterus appears bumpy and is a deep red color. The side that was facing your baby looks shiny and grayish.

How big is the placenta?

On average, a mature placenta is almost 9 inches (22 centimeters) long. It’s about an inch thick at the center (2.5 centimeters). It typically weighs a little over 1 pound (500 grams).

All these measurements can vary. Placentas can be bigger or smaller.

Where does the placenta form?

The placenta can form anywhere on the inner lining of your uterus. It develops wherever the fertilized egg implants into your uterine wall. Some positions of the placenta are:

• Posterior placenta: On the back wall of your uterus (closest to your spine)
• Anterior placenta: On the front wall of your uterus (closest to your abdomen)
• Fundal placenta: At the top of your uterus
• Lateral placenta: On the right or left wall of your uterus

You may also have a low-lying placenta. This means it’s low in your uterus and covers part or all of your cervix.

Does the placenta move?

The placenta appears to move only because your uterus expands as the pregnancy continues. Your healthcare provider will look at the location of your placenta during your 20-week anatomy ultrasound. They’ll check if its position (like a low-lying placenta) may cause complications.

Most placentas “move” to the top or side of the uterus by 32 weeks of pregnancy. It’s common to have a placenta that transitions upwards and away from your cervix as the fetus gets bigger.

What are the abnormalities of the placenta?

An issue with your placenta can cause issues for you and your baby. Some placenta complications include:

• Placenta previa: The placenta covers all or part of your cervix.
• Placenta accreta: The placenta attaches too deeply to the wall of your uterus.
• Placental abruption: The placenta separates from your uterus too early.
• Placental insufficiency: The placenta isn’t providing enough nutrients or oxygen to the fetus.
• Retained placenta: Part of the placenta stays inside your uterus after delivery.

The placenta can also form in different ways. Some include:

• Bilobed placenta: This is a placenta with two roughly equal-sized lobes separated by a membrane. It affects about 2% to 8% of placentas.
• Succenturiate placenta: One or more lobes of the placenta develop apart from the main placenta. This affects about 5% of placentas.
• Battledore placenta: The umbilical cord is at or near the edge of the placenta rather than in the center. This is more likely to happen in twin pregnancies.

Each of these conditions has different risk factors and symptoms. This is why it’s important to go to all your prenatal appointments. Tell your provider if you feel like something’s wrong.

Pregnancy conditions related to the placenta

The placenta may also contribute to other pregnancy-related conditions, including:

• Gestational diabetes: Placental hormones cause insulin resistance. This can cause gestational diabetes.
• Preeclampsia: Some researchers think this may happen due to a problem with blood supply to the placenta.

What are the most common signs of a placenta disorder?

Bleeding from your vagina is the most common sign that there’s a problem with the placenta. But this kind of bleeding has other causes, too.

Other possible symptoms and signs include:

• Abdominal pain
• Contractions
• Decreased fetal movement
• The fetus is measuring small based on its gestational age (intrauterine growth restriction)

Talk to your pregnancy care provider as soon as possible if you have any unusual pregnancy symptoms.