10 Incredible Facts About the Placenta

The placenta is mysterious but mighty. The only transient organ in the body (meaning it grows, then leaves), belonging to both the mother and the fetus, the placenta is a major part of our little corner of class Mammalia. The organ grows in the uterus and hooks into the mother’s body to provide the fetus with oxygen, blood, and nutrients. Although scientists have plenty of questions about it—there’s no way of studying it as it develops—what we do know proves how amazing the placenta is.

1. THE PLACENTA FUNCTIONS AS NUMEROUS ORGANS IN ONE.

Shaped like an umbrella over the baby, the placenta functions as multiple organs in one [PDF]. It acts as the baby’s lungs by providing oxygen from the mother, as its kidneys by filtering out waste product, and as its gastrointestinal and immune system by delivering nutrients and antibodies. It keeps the mother’s blood supply separate from that of the fetus, while also helping pass nutrients from the former to the latter. The placenta is what allows the fetus grow to a healthy size before birth.

2. MAMMALS AREN'T THE ONLY ANIMALS THAT MAKE PLACENTAS.

Although some amphibians give live birth [PDF], it’s incredibly rare for them to develop true placentas. That’s one thing that makes skinks so unique. A few species of these lizards have almost no yolk in their eggs, so the embryo needs to get nutrition from the mother. But oddly, despite appearing in a few different species of skink, the placentas are evolutionarily unrelated to each other, indicating that it might have evolved three separate times.

3. THE PLACENTA COMMUNICATES WITH THE MOTHER USING BITS OF ITSELF CALLED EXOSOMES.

One of the biggest difficulties for doctors is that the placenta can’t be monitored for growth or invasiveness through the mother’s pregnancy. If the blood vessels don’t develop correctly, they limit the blood flow to the placenta, which can result in preeclampsia, or the placenta could be penetrating too deeply into the mother’s uterine wall (or, more rarely, organs), a condition called placenta accreta. But researchers recently discovered that exosomes (tiny vesicles secreted by the body’s organs) released by the placenta might offer a noninvasive way of monitoring the placenta’s growth. The study found that at delivery, the concentration of placental exosomes correlated with the weight of the placenta. There’s more work to be done before a reliable blood test can be developed to monitor the mother’s placenta, but it could someday help doctors save women who develop dangerous conditions like preeclampsia.

4. WOMEN WHO LIVE AT HIGH ALTITUDES MAKE MORE EFFICIENT PLACENTAS.

“The evolutionary pressure on placentas is huge. It’s an evolutionary pressure much more serious than a minor tweaking of your bone structure,” says Stacy Zamudio, senior scientist and director of research at Hackensack University Medical Center. To show just how sensitive the placenta is to environmental forces, Zamudio studied indigenous women living at high altitudes in the Andes. She found that their placentas resulted in larger babies than women who had more recently moved to high altitude communities. The women whose communities had lived at high altitude for generations were more efficient at drawing oxygen from the thin air, which in turn changed the development of their placentas.

5. THE PLACENTA GIVES THE MOTHER'S IMMUNITY TO THE FETUS.

Human mothers and other higher primates (like apes and chimpanzees) start transferring their immunity to the fetus through the placenta, so that the babies are born with double the concentration of blood antibody as their mothers. This means the infants have up to six months of immune protection after birth. After that, they have to start generating their own new antibodies.

6. OBESITY HAS NUMEROUS NEGATIVE EFFECTS ON THE PLACENTA.

Perhaps unsurprisingly, women who are obese at the time of pregnancy have an increased risk of numerous placenta-related complications. They’re more likely to suffer from preeclampsia and gestational diabetes, their placentas are less effective at transferring iron from the mother to the fetus, and the higher weight increases inflammation in the placenta. All of this means the baby is born with an increased risk of developing metabolic diseases later in life, such as diabetes, cardiovascular disease, and certain cancers.

7. EATING THE PLACENTA HAS NO KNOWN HEALTH BENEFITS—BUT PEOPLE DO IT ANYWAY.

You may have heard about the sudden popularity of mothers eating their placentas after giving birth (celebrity practitioners include everyone from Kim Kardashian to January Jones). It’s a practice known as placentophagia, and plenty of other mammals do it, though that doesn’t mean it’s helpful to Homo sapiens. The women who choose to consume their placenta—raw, cooked, or in powdered capsules—claim it works as a preventative against postpartum depression or promotion of lactation, but a review of current studies found no evidence for any of the claims.

8. DIFFERENCES IN THE PLACENTA ACROSS MAMMALS MIGHT BE RELATED TO RISK OF INFECTIONS.

Although mammals are united in their ability to grow placentas and give live birth, there are vast differences in the placentas of each species. The placentas come in different shapes and sizes, and gestation periods vary from around 12 days in the American opossum to 22 months for the African elephant. Then there’s the issue of how deeply the placenta penetrates the mother. Some placentas are highly invasive (meaning they hook much more thoroughly into the mother’s blood vessels) while others attach only minimally. Although scientists aren’t quite sure about the origin of these differences, one hypothesis is that species most at risk of infection after birth have more invasive placentas so that the mothers can pass immunity on to their offspring.

9. EACH TIME A WOMAN GETS PREGNANT, A NEW PLACENTA GROWS.

Depending on how much you know about the placenta, it will either be obvious or counterintuitive that the mother grows a new one every time she’s pregnant. “Each time the placenta is new. It is genetically distinct to that pregnancy and that child,” says Catherine Spong, acting director of the National Institute of Child Health and Human Development. In the case of twins, it’s a bit more complicated: Identical twins usually share a placenta while fraternal twins usually have their own separate placentas.

10. SCIENTISTS STUDY THE PLACENTA TO UNDERSTAND CANCER.

Unlike organ transplants and other foreign objects entering the body, the placenta has the unique ability to develop without the immune system going into attack mode. And when it develops normally, the placenta stops infiltrating the mother’s body before it does any harm. This makes it a potentially invaluable source of information for cancer researchers, who want to better understand how tumors grow without being stopped by the immune system. One study that compared abnormal placentas (which caused preeclampsia) to the genetic profile of a tumor found the two shared a number of traits. Basically, the placenta is similar in many ways to cancer, and understanding it better could help researchers combat the growth of cancerous cells.