I polled my Twitter followers recently to find out what they wanted me to cover, and heard back a resounding "CONTRACEPTIVES!" So first I am going to re-post a series I wrote on my lab blog in July of 2009, with significant editing and updating. I think after these reposts I'll have a better idea of where it would make sense for me to contribute more, if at all. This is post two of five. Part one can be found here.
What is a normal menstrual cycle? Are you normal? Am I? Women spend a lot of time worrying about this, and most of them seem to assume they fall outside of normal. So, I'd like to spend a little time unpacking the concept of normal reproductive functioning in the second part of this 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). This led me to wonder what it means to be biologically normal. That offhand, and troubling, statement about there being "nothing biologically normal" about month-long cycles and frequent menses, makes an entire population of women feel as though they are, then, biologically "abnormal." I don't think this is useful.
My understanding of biologically normal is that the body varies and is responding to its environment in a way that we would consider to be adaptive. It is normal for the female reproductive system to allocate resources in response to its environment.
Women from industrial populations are 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 introduce the idea that American physiology is not the global standard that we should use to evaluate all human populations. However, any body that responds appropriately to its ecology is, by the definition I've always learned, normal.
The reason Stearns and Koella (2008), and Eaton et al (1994; 2002), and others have been advocating continuous hormonal contraceptive use in industrial populations is that it may decrease reproductive cancer rates. The relatively higher incidence of reproductive cancers in industrial populations is a consequence of our flexibly responsive bodies being in an environment of low energy constraint. 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 plenty of women 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 (1997) has a great analysis of this and comparison between populations: 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 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 outside it, as in 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 want to complicate things further and ask if it is actually true that continuous oral contraceptive use would actually reduce reproductive cancer risk, and if so, at what age should it be administered? Currently, I'm not convinced that getting women on to oral contraceptives for their entire reproductive years is wise. But that's for another post.
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