Why Do Humans Menstruate?

who-controls-menstruatoinHave you ever wondered why humans have periods while most other mammals do not? I know that I have. It seems strange that the only other mammals, besides humans, that menstruate are apes, monkeys, elephant shrews, dogs and some types of bats. In recent years, researchers, like Deena Emera of Yale University, have been working to explain this phenomenon and they have come up with intriguing explanations.

Who Controls The Womb: Mother or Embryo?

According to Emera, whether a mammal menstruates depends on how much control she has over her womb versus how much control the embryo has over it.

Embryos can only implant if the uterus has a thick lining. In humans, and other mammals who menstruate, changes in the lining of the uterus are controlled by the female or, more accurately, the hormone progesterone. This is called spontaneous decidualization and allows the female’s body to control, through her hormones, whether she is able to get pregnant. Since the thickened uterine lining is not always used to support an embryo, species who exhibit spontaneous decidualization shed the lining, resulting in a period.

In contrast, in other non-menstruating mammals, uterine changes are controlled by the embryo, with the uterine lining thickening in response to pregnancy. In these animals, since the uterine lining is always used for a pregnancy, it does not need to be shed and there is no resulting period.

The question now becomes: why do some mammals control their own wombs, through spontaneous decidualization, while others have their wombs controlled by embryos?

Aggressive Fetuses and Genetic Abnormalities

Emera suggests two possible explanations for why humans and some other mammals exhibit spontaneous decidualization.

The first explanation deals with the inherent conflict between a mother’s body and a developing fetus. In all mammals, fetuses burrow into the uterine lining, seeking nourishment; however, the fetuses of some species are more aggressive than others. In some animals, like livestock, fetuses sit on the surface of the uterine lining, doing very little burrowing. In primates and humans, fetuses are more aggressive, burrowing deeply into the lining to get as much nourishment as possible. Emera suggests that spontaneous decidualization evolved as a means to protect the mother’s body from these more aggressive and invasive fetuses.

The second explanation suggests that spontaneous decidualization developed as a way to get rid of embryos with genetic abnormalities. The embryos of humans and other primates are more prone to genetic abnormalities. Once the uterine lining thickens, its cells can detect genetic problems and remove defective embryos, before the mother uses valuable resources on an embryo that will not develop into a healthy fetus. This is especially important in species, like humans and apes, that have long pregnancies and tend to only have one baby at a time.

While both explanations have only been advanced as possible explanations, it is clear that researchers are getting closer to explaining the mysteries of menstruation.

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Shannon Flynn

Shannon Flynn is an attorney and freelance writer in Columbus, Ohio. She received both her bachelor's degree in Psychology and law degree from The Ohio State University. In her spare time, she enjoys dancing, doing Pilates, playing with her two cats, and spending time with her fiancé.