Inflammation of the Placenta (Placentitis): Information for Pregnant Women

If you read a lot of health content, including posts here on The Pulse, you probably know that pretty much any part of the body can become inflamed, in which case we add the suffix itis to the name of that body part to indicate the inflammatory condition. You also may know that, usually, inflammation of an organ or other body part is the result of an infection, although there also are inflammatory conditions that are autoimmune, meaning that a person’s own immune system attacks the body or parts of the body. We have discussed the placenta from several different angles. These include abnormalities of the location of the placenta (such as placenta accreta and placenta previa), as well as placental abruption (abruptio placentae) in which the placenta actually detaches from the uterine wall. We have discussed the emerging technology of the artificial placenta and how it will lead to a full artificial womb. Topics on The Pulse have even included the placenta as food (not just as mother cats’ food but possibly even for human parents). In the context of the COVID-19 pandemic, however, the placenta moved to center stage when we debunked a popular myth that claimed that COVID-19 vaccination could harm the placenta, and, prior to that, in a very popular article on how this organ is affected when a mother becomes infected with SARS-CoV2 (the virus that causes COVID-19). Today’s story relates especially to the latter example, because, even though the placenta is a temporary organ, it is still an organ that can become infected, resulting in what doctors call placentitis, meaning inflammation of the placenta.

Just to review what we mean when we talk about the placenta, the placenta is a temporary organ that serves as a boundary between the blood of the mother and the blood of the fetus. Within the placenta, there are small blood vessels known as capillaries. These capillaries carry the fetal blood (and prior to that embryonic blood) into close contact with capillaries carrying maternal blood. Because the embryo/fetus has special hemoglobin in its red blood cells (RBCs) that attracts oxygen more strongly than hemoglobin of an adult, the placenta is where oxygen moves from the maternal to fetal blood while the RBCs themselves remain on their appropriate side of the circulation (fetal RBCs on the fetal side, maternal RBCs on the maternal side). At the same time, the placenta is also where the waste gas known as carbon dioxide moves from the fetal blood to the maternal blood (some carbon dioxide is carried by hemoglobin, but most of it converted to a compound called bicarbonate, which is dissolved directly in the water component of the blood. Thus, from the perspective of the fetus, the placenta is a place where you send your carbon dioxide to get rid of it and where you receive oxygen to freshen your blood. In other words it serves the function that the lungs serve after birth, but for much of embryofetal life, it also does a lot of what the baby’s liver and kidneys will do later; it filters out things that could be toxic. There is a kind barrier between the maternal and fetal capillaries, that allow certain things to move from the material blood to the fetal blood. So in a sense, the placenta is a jack of all trades. It is vital to the development and survival of the fetus up to the point of birth.

While placentitis is an issue that you will find is discussed more in veterinary medicine, such as in connection with horses, than in humans, various infections agents can indeed cause placentitis in humans. As one timely example, it has been known for about two years that COVID-19 can produce abnormalities in the placenta, including filling its specialized blood vessel system with numerous blood clots, but a newly published study from scientists at the Massachusetts General Hospital (MGH) in Boston drives home this idea. In examining dozens of placentae from women who gave birth while suffering from COVID-19 or subsequent to suffering from the disease, researchers found that placentitis often does occur as a complication of infection with SARS-CoV2. Moreover, whereas placentitis from COVID-19 does not always include numerous blood clotting in the placenta, the MGH study suggests that such clotting can be expected in a placenta that becomes inflamed in a woman who is not vaccinated against the virus. In fact, just about all of the cases of placentitis occurred in women who were not vaccinated against COVID-19. So this is one more reason that you can add to the many that you already know for being vaccinated against this devastating pandemic.


David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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