As you read this article, typed on a laptop and accessed through the internet, it’s easy to appreciate that humans hold a unique place in the animal world. Humans have intelligence, dexterity, and a drive to create that sets them apart. But aside from the traits that allow for the creation of symphonies and cathedrals, there are other traits that stand out in the animal world, especially in regards to reproduction.
For one, female humans ovulate in year-round cycles instead of only during a specific time of year, and ovulation is “hidden” or without obvious outward signs (just more subtle signs that can be tracked through FAM!). Female humans also have periods, something only a few species, like apes, eastern-hemisphere monkeys, some bats, and the adorable elephant shrew also have. Other species either don’t experience a menstrual cycle or simply reabsorb the uterine lining. It’s an exclusive club, but… why does it exist? Why do women have periods?
Evolutionary Biology 101, or how polar bears help us understand why women have periods
In my experience with biology research, I’ve found that a lot of “why” questions are really “how” questions. In understanding a particular trait, we want to understand how the trait or process works and how this trait came to be.
An example I like to use for this is polar bears. Polar bears and grizzly bears are closely related, branching from each other a mere six hundred thousand years ago, as reported by National Geographic. To answer the question, “Why do polar bears have white fur?” is relatively simple; white fur blends in with white snow.
As a researcher, “How did polar bears get white fur?” seeks to explain further. Ancestors of polar bears likely had brown fur but some bears with white, or at least lighter-colored, coats appeared occasionally as a result of recessive mutation. In temperate time periods, these bears would lead similar lives to their darker-furred peers. However, during ice ages and times with scarce food, light fur would provide enough of an advantage that these bears would be more likely to survive to reproduce than bears without this trait. Over the millenia, genes for dark fur would be gradually removed from the population during times of scarcity until the entirety of these tundra-living bears had white fur.
The question, “Why do women have periods?” is relatively straightforward: women have periods to shed the old uterine lining to prepare a new one. Let’s explore how this trait may have come to be and what advantage it provides.
Women need periods because of decidualization…
First, let’s do a quick recap of what actually goes on during a period. A period is the shedding of the outer layer of the endometrium, which lines the uterus. This outer layer goes through important structural changes during the menstrual cycle called decidualization. Decidualization is the process of the endometrium preparing for pregnancy and becoming the decidua: it involves changes in endometrial cells, introduces special white blood cells that protect the fetus from the mother’s immune system, and will later alter the setup of the surrounding blood vessels.
This process is initiated by the upsurge in progesterone after ovulation, but only in species that menstruate. Other mammals’ bodies do not prepare for pregnancy until an embryo is already present. If pregnancy does not occur (in menstruating species) the endometrium that started prepping for pregnancy is shed or reabsorbed.
…since human mothers need a two-week head start to prepare for baby’s implantation
As we mentioned, menstruating species undergo decidualization each cycle in order to prepare for pregnancy in advance. The reason this is so important in humans especially is that our embryos implant deeply. This creates a delicate dance between what mom’s body needs and what the embryo needs. The embryo needs to attach securely and access nutrient-rich blood as soon as it can. However, the mother’s body has to set healthy boundaries around this enthusiastic implantation approach.
Thus, the mother’s body gets a two-week head start on preparing the endometrium to thicken the lining and prevent the embryo from burrowing too deeply into the uterus. Achieving this deep-dive into mom’s tissue is also a crucial testing ground the embryo must pass.
Successful embryonic implantation is a key reason why women have periods
Research in mice has demonstrated how the uterus and embryo must signal to one another for implantation to occur [1]. If the newly conceived blastocyst is not viable, as some research indicates is the case nearly half the time, implantation will not occur [2].
While the creation of this well-prepared, thick uterine lining is beneficial when a viable pregnancy occurs, it creates a dilemma when no pregnancy, or an early pregnancy loss, occurs. In early pregnancy loss, the only way for the embryo to leave the body is for the body to shed the uterine lining (lest it risk infection). Even when no pregnancy occurs, this thick, specialized endometrium cannot be reabsorbed like in other mammals and must be shed.
Why can’t the body just have this specialized endometrium all the time?
For a lot of women, periods are no walk in the park. It might sound like overkill for the body to replace the endometrium month after month, especially if there’s no chance of pregnancy. One theory is that it actually takes less energy and resources to replace the endometrium rather than to maintain it. For women whose reproductive history has allowed them to sidestep many years of menstruation (i.e. having many children with long periods of lactational amenorrhea), this theory has some merit. For women who menstruate for all or most of their reproductive years, especially with heavy or painful periods, this theory starts to sound a lot less compelling!
Another theory is that it’s risky business to keep endometrial tissue around for too long. The process of decidualization prompts the endometrial cells to transform and grow rapidly, a process that, if it gets out of control, can turn cancerous.
The couch-surfer vs. deluxe guest room model of childbearing
Overall, the concept of decidualization simply provides more details about what we already know about the menstrual cycle; namely, it allows the body to prepare for pregnancy. Importantly, while other species are content to let an embryo crash on the couch, so to speak, the human body needs to craft a plush, deluxe guest room in advance. This is because human embryos are voracious guests who settle in deeply, and form deeply-rooted placentas to get all the nutrients their complex bodies and brains will need.
Everyone deserves to know why women have periods
As explored in our Reasons Women Need Periods series, this recurrent, hormone-driven process affects the body as a whole. As someone who loves biology, learning answers about periods leads me to more questions, especially regarding how so many parts of the body rely on the fluctuating balance of estrogen and progesterone the menstrual cycle creates to develop and function. The body is fascinating, and one way to start learning about how your body works is through fertility awareness and discovering how this complex system works for you personally.
