Reviewed by: BD Editors

Placenta Definition

The placenta is an organ which is responsible for nourishing and protecting a fetus during pregnancy. It is unique in that it is a temporary organ; it grows alongside the fetus during pregnancy, and then is expelled along with the fetus at birth. The placenta is also sometimes called “afterbirth,” as it is expelled through the vagina after the fetus has been delivered.

The placenta performs myriad functions to support fetal development, including facilitating blood flow, gas exchange, waste elimination, and serving as a protective barrier for the fetus against any infections the mother experiences during pregnancy.

The placenta is unique in that it is an organ which arises from the tissue of two genetically distinct organisms; part of the placenta develops from the tissue of the mother’s uterine wall, while another part develops from the fetus’ own tissue. After the blastocyst which will develop into the fetus makes contact with the uterine wall, blastocyst and maternal tissue grow together to form a single, cooperating organ that links the two together.

Mammals who give birth to live, fully-developed young, rather than laying eggs or carrying underdeveloped offspring in pouches, are often called “placental mammals.” The evolution of the placenta is one of the primary characteristics shared by all mammals except for marsupials and egg-laying mammals such as the platypus.

Some marsupials – close cousins to placental mammals like cats, dogs, and humans – have a primitive placenta in which blood vessels grow throughout the protective membranes surrounding the embryo. It is possible that placentas as we know them may have evolved from similar primitive structures in found the common ancestor of marsupials and placental mammals.

Pregnancies in which the placenta does not develop properly typically miscarry. This can occur either because of problems with the development of maternal uterine tissue, or because of problems with development of the fetal placental tissue. Fetuses which suffer from chromosomal problems may not be able to form a proper placenta and may miscarry, most often in the first trimester.

Many animals and some human cultures make a practice of eating the placenta after it is delivered. Scientists are divided on whether this is a good idea for humans. Some say it may contain valuable nutrients and even hormonal components that can be of assistance to the mother after pregnancy; others say that this practice may carry a risk of spreading infectious diseases, and that no benefits to eating the placenta have been proven in humans.

Function of Placenta

The placenta acts as a lifeline between the mother and fetus, ensuring that the fetus gets what it needs from the mother’s body to survive. At the same time, it acts as a protective barrier, shielding the fetus from some maternal infections. The functions of the placenta include:

  • Allows gas exchange so the fetus gets enough oxygen
  • Helps the fetus get sufficient nutrition
  • Helps regulate the fetus’ body temperature
  • Removes waste from the fetus for processing by the mother’s body
  • Filters out some microbes that could cause infection
  • Transfers antibodies from the mother to the fetus, conferring some immune protection
  • Produces hormones that keep the mother’s body primed to support pregnancy

The placenta, then, essentially serves the function of several organ systems for the fetus, since the fetus is unable to eat, breathe, or eliminate waste itself while it is inside the womb!

Unfortunately, the placenta is not fool-proof as an infection barrier, and some fetuses do become infected with diseases contracted by their mother. Toxins of sufficiently small molecular size can also pass through.

That’s one reason why pregnant women are advised to avoid all possible sources of disease and toxins – diseases and toxins which may not be dangerous at all to adult women may be devastating to a developing fetus.

How Does the Placenta Work

When it is delivered, the placenta looks like a flat, round organ that is suffused with thick blood vessels. The fetus’ umbilical cord attaches to one flat surface, while the reverse surface grows out of the mother’s uterus during pregnancy.

The placenta works mainly by allowing substances to be exchanged between maternal and fetal blood. This allows the fetus to obtain nutrients, oxygen, antibodies, and other vital substances without having to share the mother’s blood supply directly.

This is vital because fetuses do not always have the same blood type as their mother, and direct mixing of the bloodstreams could cause the mother’s immune system to attack the fetal blood supply. Even with the placenta separating the two, problems are occasionally caused by maternal antibodies attacking fetal blood supplies. Some women receive vaccines or other treatments to stop that form happening.

The diagram below shows how the fetal blood vessels infiltrate the placenta. It also shows how the mother’s arteries permeate the placenta. The placental tissue in between the two acts as a sort of filtration system, preventing most cells from passing through the barrier while allowing substances such as nutrients, antibodies, and gases to do so:


Eating the Placenta

In recent years, the topic of placenta-eating has been in the news a great deal. Many celebrities have contracted with companies that promise to turn their placentas into pills or food, and touted it as a move that has great health benefits. But many doctors have cautioned that there’s no evidence that placenta-eating confers real health benefits, and that eating your placenta may actually make your baby’s health worse.

In nature, animals often eat their placentas. There’s good reason for this: in nature food is often scarce, and the placenta is rich in protein, iron, and other nutrients that can be difficult to procure in the wild. That means that placenta-eating is often worth the risk for animal moms who just gave birth, and now need to provide nutritious milk for their young.

However, for humans, the risk of disease may outweigh any benefits of placenta-eating. Because the placenta serves as a filter to prevent harmful bacteria and viruses from reaching the baby, it can contain bacteria from infections the mother had during pregnancy.

Even if these pathogens aren’t harmful to the mother – some viruses and bacteria hardly bother adults – they can still be passed on to the newborn baby through breast milk, if the mother consumes an infected placenta. Cases have been recorded of babies becoming sick with bacterial infections which were later traced to their mother’s placenta supplements.

For that reason, many doctors counsel that the placenta is just like any other human tissue – it should not be eaten by humans, because doing so could spread disease.

Because the placenta does not qualify as a medication, health food and birthing companies that promise a “healthy experience” if you pay them to prepare your placenta are often not subject to the same regulations for safety and effectiveness as medications are.

In conclusion, just because some famous moms have done it doesn’t mean it’s a scientifically supported idea!


1. Why are mammals such as dogs, cats, and humans called “placental mammals?”
A. Because the placenta is the evolutionary adaptation we all share, while monotremes, marsupials, and non-mammals do not.
B. Because all placental mammals have placentas, while only some non-mammals have placentas.
C. Both of the above.

Answer to Question #1
A is correct. Only placental mammals have fully developed placentas, while non-mammals and older mammal lineages such as monotremes and marsupials do not.

2. Which of the following is true of the placental filtration system?
A. It is able to filter out all diseases and toxins, protecting the fetus.
B. It allows nutrients and oxygen to pass through, nourishing the fetus.
C. It is able to filter out some diseases and toxins, but not all of them.
D. Both B and C.

Answer to Question #2
D is correct. The placenta allows nutrients, oxygen, and other beneficial substances to pass from the mother to the fetus. It screens out some bacteria and toxins, but is unfortunately not able to screen out all of them. That’s why pregnant women are counseled to avoid potential sources of toxins and disease.

3. Which of the following is NOT true of placental anatomy?
A. It contains many blood vessels from both the mother and fetus.
B. It is composed of both maternal and fetal tissue.
C. It allows the mother’s blood to flow into the fetus, nourishing it.
D. None of the above.

Answer to Question #3
C is correct. The placenta separates the maternal and fetal blood supplies, to prevent the mother’s immune system from attacking fetal blood cells. However, it does allow some substances such as nutrients, gases, and antibodies to be exchanged.


  • Slater, D. (2017, April 03). The Myth Of The Placental Barrier. Retrieved July 08, 2017, from
  • 10.2 Development of the placental villi. (n.d.). Retrieved July 08, 2017, from
  • Should I Eat My Placenta? (n.d.). Retrieved July 08, 2017, from

Cite This Article

MLAAPAChicago Editors. "Placenta." Biology Dictionary,, 25 Jul. 2017, Editors. (2017, July 25). Placenta. Retrieved from Editors. "Placenta." Biology Dictionary., July 25, 2017.

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