The placenta: What it is and how it works

What is the placenta?

The placenta is a pancake-shaped organ that develops within the wall of your uterus and connects to your baby though the umbilical cord. By the end of pregnancy, it grows to be about 9 inches in diameter and about an inch thick at the center.

What does the placenta do?

• It delivers oxygen and nutrients (such as vitamins, glucose, and water) from your body to the baby's, and processes the waste products from your baby.
• It produces the hormones that help your baby grow and develop.
• It allows antibodies to pass to your baby from your bloodstream. These antibodies protect against certain bacterial infections and viral illnesses, like diphtheria and measles, until after your baby is born and old enough to get their first vaccinations.

When does the placenta form?

The placenta starts forming as early as six days after fertilization. At this point your baby-to-be is a hollow ball of several hundred cells called a blastocyst. The blastocyst attaches to the lining of the uterus, usually near the top. This is called implantation, and it's completed by day 9 or 10.

The wall of the blastocyst is one cell thick except in one area, where it's three to four cells thick. In the thickened area, the inner cells develop into the embryo. The outer cells throughout the embryo burrow into the lining of the uterus and develop into the placenta.

The placenta produces several hormones that help maintain the pregnancy, including human Chorionic Gonadotropin (hCG), which prevents the ovaries from releasing eggs and stimulates the ovaries to produce estrogen and progesterone continuously. (Home pregnancy tests measure levels of hCG in your urine.)

The placenta is fully formed by 18 to 20 weeks but continues to grow throughout pregnancy. At delivery, it weighs about 1 pound.

How the placenta works

The placenta connects to the umbilical cord through thousands of microscopic "fingers" of tissue (chorionic villi) containing a network of blood vessels that connect to the embryo's circulatory system. The villi are formed by 6 weeks of pregnancy, and your blood fills the spaces around them by 12 weeks. When your blood comes in contact with the villi, nutrients are exchanged for waste through the villi walls.

The villi also act as a filter, preventing some viruses and bacteria from reaching your baby, while allowing molecules of everything you ingest as well as antibodies and gases to pass through.

How do babies get nutrients in the womb?

Your baby needs nourishment from the get-go. Until the placenta is developed enough to provide nutrients, your baby receives nutrients from the lining of the uterus, called the endometrium. (These same endometrial cells will quickly multiply to form the placenta.)

In the early weeks of pregnancy, glands in the uterine lining secrete glucose, which it stores as glycogen. At first, this glycogen is the only source of nourishment for your baby.

At about 8 to 12 weeks into pregnancy, the placenta takes over as a nutrient source for your baby. (And at this point the uterus stops secreting glycogen.) Here's how it works:

1. When you eat, the food travels to your stomach, where it's broken down (digested) into glucose, fats, vitamins, minerals, and protein.
2. The nutrients are absorbed into your bloodstream and travel to the placenta.
3. Blood vessels in the umbilical cord pass the nutrients from the placenta to vessels that flow from the umbilical cord to the baby. At this point, your baby will begin to gain weight more quickly. The umbilical cord also returns waste products from your baby to the placenta and into your circulation for elimination.

Wondering how fast the food you eat reaches your baby? It depends on how quickly the food is digested and enters your bloodstream. Some foods may take several hours, while substances like caffeine can enter your bloodstream and cross the placenta in a very short time. For more information, watch our video on how food reaches your baby.

Harmful substances that cross the placenta

Along with all the good things that are transferred to your baby, some harmful ones can cross the placenta too. These include alcohol, nicotine, and illegal drugs as well as some medications.

No safe level has been established for alcohol consumption during pregnancy, and experts recommend not drinking any amount while you're expecting. The same goes for smoking and using illegal drugs. Although marijuana is legal in some states, experts recommend against using it during pregnancy.

Use caution when taking prescribed and over-the-counter medications during pregnancy because most cross the placenta. Although many medications are considered safe for your baby, a few are known to cause birth defects. And for many drugs, there simply is not enough research to know their effects on an unborn baby. Your provider can help you determine whether the benefits of a certain drug outweigh the risks in your case.

Although the placenta acts as a barrier to most bacteria, some viruses and small bacteria can cross it. Many have no effect, but others can be harmful to a developing baby. That's one reason why it's important to get all recommended vaccinations before and during pregnancy, including a flu shot and a Tdap vaccine to protect against tetanus, diphtheria, and pertussis (whooping cough).

Both the flu and pertussis can be life-threatening for babies, and getting these vaccines while you're pregnant enables antibodies to pass through the placenta that will protect your newborn from catching these illnesses. Getting your COVID vaccine and boosters will help protect you from infection and will also provide your baby with antibodies through the placenta (and your breast milk, if you breastfeed).

Here are a few other things you can do to promote a healthy placenta and a healthy baby:

• Go to all your prenatal checkups and work with your healthcare provider to manage any health conditions, such as high blood pressure, which can cause problems with the placenta.
• Don't smoke or use drugs. Placental problems, like placental abruption, are more common in women who smoke, use illegal drugs (like cocaine and heroin), or abuse medications (like opioids).

Delivering the placenta

A few minutes after your baby is born, the placenta detaches from the wall of your uterus and is delivered through your vagina. This is called the "afterbirth."

You'll have a few contractions, but this stage of childbirth usually lasts only about five to 30 minutes and feels more like menstrual cramps than labor pains. (If you're having a c-section, your doctor will remove the placenta manually.)

If you want, and if you've made prior arrangements with your provider, you can save the placenta. Some women believe that encapsulating and eating the placenta after birth helps them recover faster, though there's no scientific evidence to support this claim.

Also note that some experts think eating the placenta may be harmful, as there are potential risks, including infection and heavy metal contamination. Be sure to discuss this topic with your provider if it's something you're considering.

Learn more:

Placenta previa

Placenta accreta

Slideshow: Fetal development, week by week