March (Women’s History Month) is generally the month in which I feel most at ease to talk about the things that interest me. With that in mind:
A subset of placental mammals (of which you are one) have spontaneous decidualization (~2%). What does that mean and who cares? Spontaneous decidualization is when your uterus prepares for pregnancy on a regular basis regardless of an implantation event. You will recognize this as your menstrual cycle. You shed your endometrial lining and bleed, in a relatively regular manner for years and years, because your body is preparing for pregnancy every month regardless of whether you are. For evolution to select for such an energetically intense process, shedding endometrial tissue month after month, is nothing short of remarkable.
Further, spontaneous decidualization and menstruation have been observed in fruit bats and elephant shrews, and so it is reasonable to infer that menstruation arose independently multiple times. (I’ll keep this LinkedIn lite and save you the discussion of parsimony, evolutionary convergence and comparative anatomy.) The takeaway: a trait that develops independently 4 times is likely to be an adaptive trait which provides some advantage to reproductive success.
What could be so advantageous to “staying ready” for pregnancy? Many fascinating theories exist to partially answer this question, but the somewhat unsatisfying conclusion to date is: we need more data to reach a consensus.
A different, natural question follows: if spontaneous decidualization is beneficial, are there any consequences to stopping your cycle for extended periods of time? Birth control in general has been transformative to the lives of women, from increasing agency to enabling economic freedom. The use of long-acting reversible contraception (LARC) has provided a highly effective means of birth control and its use has increased substantially since entering the market.
And while studies show a return to fertility after LARC use, these studies are focused on women who have been pregnant before, and who are using copper IUDs where the duration of use is on average less than 2 years. How will 6- to 10-years of progestin IUDs affect time-to-conception for people who have never been pregnant? And are these questions even more relevant as the average age of childbearing increases, people start the use of LARCs at a younger age and are on this method for longer periods of time?
As the paper in the comments highlights, with new molecular tools to evaluate changes in the endometrium, our ability to track and answer these questions will improve over time. It is precisely because birth control is such a critical tool for women that we need to be rigorous about seeking more data, and asking more questions.
While this image was created with the assistance of AI, the content of this post was not.