Parts I, II, III, IV and V.
So I've covered a bit of my bias against hormonal contraception ads, and the basic no-nos. But one of the things you may have heard a lot lately is that women don't "need" a period, or even that it is "useless." I'd like to spend a little time unpacking this in the second part of my series.
What is normal?
Recently, in the beginning of an evolutionary medicine volume, I read in the editors’ opening comments that there is “nothing biologically normal” about monthly menses, as a way to put forward the idea that women should take continuous oral contraceptives (Stearns and Koella 2008, p. 4). Nothing biologically normal? Whether we like it or not, frequent menses in the United States is biologically normal, due to the fact that we eat a lot and don’t move around much at all. We’re at the far end of the spectrum of variation in reproductive function, but we have not fallen off the end of the continuum. On the one hand, I appreciate the attempt of the authors to try to shake things up and introduce the possibility that American physiology is not the global standard, but any body that responds appropriately to its ecology is, by definition, normal. Does this appropriate and evolutionary response to environment have its own consequences? Yes; you can lay the blame for the increased incidence of reproductive cancers in developed countries mostly on the flexible responsiveness and resource allocation capabilities of our reproductive systems. Your genes are important in determining your chances of breast cancer, but so are cumulated years of cheese fries and driving to work. (And before you think this is some sort of diatribe against overweight folks, plenty of normal weight folks – normal for Americans – have more unhealthy eating patterns than overweight folks. It’s not how you look but how you live, even when in broad strokes evidence can initially suggest it’s about the weight one carries.)
Here is the reason Stearns and Koella (2008), and Eaton (Eaton et al. 1994; Eaton et al. 2002), and others have been advocating continuous hormonal contraceptive use: it may decrease reproductive cancer rates. Let’s take a step back and first understand the context under which the human female reproductive system evolved: once upon a time we were eating less and moving more. Age at menarche (that’s when we get our first menstrual period) used to be much later, menses itself wasn’t particularly heavy or cumbersome, and few cycles were ovulatory (meaning that an egg is released for possible fertilization). Soon after reaching menarche (as in, within a few years) a woman has her first child. She breastfeeds intensively for the first few years, but continues to breastfeed at least occasionally for four years, maybe more. At some point towards the end of breastfeeding, or sometimes not even until breastfeeding was done, she would resume cycling, and in a few cycles likely get pregnant again.
This pattern would continue, with some variations based on miscarriages, increasing age, seasonal variation in food availability, and other issues, until the woman hit menopause. Of course, for many women, their lives ended around that point or even before, but some number of women certainly survived to be grandmothers, if observation of current forager populations is any indication. This means that for most of a woman’s reproductive life she was pregnant or breastfeeding, and cycling only occasionally. Strassmann has a great analysis of this and comparison between populations (Strassmann 1997): the punchline is that an industrialized woman of today has around 400 menstrual cycles, while our ancestors, if modern foragers are an indication, had 50-100.
Now let’s look at today’s industrialized, or developed-country woman: like men, she eats more and moves around less, largely because she is in school or working rather than getting her own food. She hits menarche earlier, and menses are more frequent and copious than her ancestors, which creates lots of tissue remodeling in the endometrium (the lining of the uterus). Many of her cycles are ovulatory, necessitating frequent tissue remodeling for the ovaries. She may cycle for years before having her first child, even decades, and with those frequent cycles come a higher exposure to endogenous (coming from within the body rather than a pill) sex steroids like estradiol and progesterone. Even if she breastfeeds for years, she will likely resume menstrual cycling sooner than her ancestors because she is better fed. She will probably have fewer pregnancies and births than her ancestors, which means more cycles in between pregnancies. She will most likely make it to menopause and beyond; because she is so much more likely to make it past menopause we are far more likely to notice the negative effects of all that hormone exposure, in the form of reproductive cancers.
So while I disagree with the idea that there is “nothing biologically normal” about frequent menstrual cycles, I certainly agree that they are not doing us any favors. But is it the reproductive system that is at fault or the lifestyle? Should we artificially suppress the system in order to promote health, or make changes to the way we live? I’m sure the answer lies somewhere in between, at least as we move towards building more sustainable neighborhoods and taking better care of our environment in developed countries.
The third part of this series will address population variation in reproductive function, and how this impacts the efficacy and side effect incidence of hormonal contraceptives.
Eaton SB, Pike MC, Short RV, Lee NC, Trussell J, Hatcher RA, Wood JW, Worthman CM, Blurton-Jones NG, Konner MJ, Hill KR, & Bailey R (1994). Women's reproductive cancers in evolutionary context Quarterly Review of Biology, 69 (3), 353-367
Eaton, S.B., Strassmann, B.I., Nesse, R.M., Neel, J.V., Ewald, P.W., Williams, G.C., Weder, A.B., Eaton III, S.B., Lindeberg, S., Konner, M.J., Mysterud, I., & Cordain, L. (2002). Evolutionary health promotion Preventive Medicine, 34, 109-118
Stearns S, and Koella J, editors. 2008. Evolution in health and disease. 2nd ed. Oxford: Oxford University Press.
Strassmann, BI (1997). The biology of menstruation in Homo sapiens: Total lifetime menses, fecundity, and nonsynchrony in a natural-fertility population Current Anthropology, 38 (1), 123-129 ISI: A1997WD24700015