Most of us are familiar with a 28-day menstrual cycle, which, divided in half, comprises the follicular phase – that’s when the dominant follicle, or egg, is growing and preparing for ovulation – and the luteal phase – when the endometrium, or lining of the uterus, is preparing for possible conception and implantation. If implantation doesn’t occur and signal pregnancy to the corpus luteum, which is the "yellow body" left behind by the ovulated egg, then it eventually degrades, hormone levels decline, and menstruation occurs. Then the whole process starts up again.

However, the frequency of such a cycle in any given woman’s life, in any given month, is pretty low. This contradicts what we learned in health class and sex ed, and sometimes the things girls and women hear from peers, parents, and doctors. Most young girls just getting their period take years to achieve regular cycles like the one I described above: this is the main characteristic of the reproductive functioning of adolescents. Yet this normal variation sets us up for a lifetime of checking our cycle, counting days, doubting that we’re like other girls, and feeling bad about ourselves, because of that early information that we should be achieving twenty-eight-day cycles regularly.

When I first became an anthropology major in college, I remember being nearly overwhelmed with data on the different factors that produce variation in women’s hormone levels and cycle lengths: our reproductive functioning is influenced by how much we eat [1], how much we exercise [2], whether we are trying to lose weight [3], and whether we are recreational athletes [4]. Some studies even show an influence of psychological stress on cycle length [5].

What also interested me was that we could measure a lot of variation between populations that appeared to be based not on genetics, but on lifestyle. Women in rural southern Poland, which is the population I study, have cycles on average about the same length as ours, maybe a touch shorter, but their progesterone concentrations are lower, our data suggest they may not maintain their endometrial thickness through their luteal phase the way women in industrial populations do [6, 7]. This is because I do my fieldwork in the summer months, when they are working long hours in the fields harvesting. Much of their work must be done by hand, and men and women work equally hard at farming. So even though their body weight and body mass index are about the same, and they eat about the same number of calories as those of us sitting at desk jobs in the US, their hard work and energy constraint is like a signal to their bodies that they should not allocate too much finite resource to reproduction.

When I learned these things in college, and then saw it firsthand in my own fieldwork and lab results in graduate school and beyond, it wasn’t all new information. Women are aware that lifestyle and other factors affect their bodies. However, the signal we tend to get is that deviation from that twenty-eight-day cycle is not a good thing. What I learned from the evolutionary lens of biological anthropology is that this variation is adaptive, that when we vary from this false norm – really an extreme that does not reflect the physiology of most women of the world – and can identify lifestyle or environmental factors producing this variation, that it means our bodies are working properly. It would not make sense for our bodies to produce cycles capable of conception all the time, if we don’t always have the attention, energy or resources to support a baby. And even if we have other reasons to not want a child that wouldn’t influence our bodies, like our jobs or our preferences, constant ovulatory cycling exposes our bodies to doses of hormones that can increase our risk of reproductive cancers [8, 9].

I’m not asking every woman to embrace painful periods, cycles so long as to make you wonder every few months if you’re pregnant, or symptoms that significantly interfere with her quality of life: I know some women are derailed by pain, discomfort or other symptoms and find regulating their cycle helps, and at a few points in my life I have too. But I am asking that we use a different frame to understand and describe our bodies. If responding to our environment is adaptive, then variation is adaptive, and variation is the real norm. The better we understand how our lifestyle and environment impact our cycles, the better we can forgive a little variation, and thank our bodies for knowing what to do, sometimes better than we do.


1. Bentley, G.R., A.M. Harrigan, and P.T. Ellison, Dietary composition and ovarian function among Lese horticulturalist women of the Ituri forest, Democratic Republic of Congo. Eur J Clin Nutr, 1998. 52(4): p. 261-270.

2. Warren, M.P. and N.E. Perlroth, Hormones and sport: the effects of intense exercise on the female reproductive system. Journal of endocrinology, 2001. 170: p. 3-11.

3. Lager, C. and P.T. Ellison, Effect of moderate weight-loss on ovarian function assessed by salivary progesterone measurements. Am J Hum Biol, 1990. 2(3): p. 303-312.

4. De Souza, M.J., Menstrual disturbances in athletes: a focus on luteal phase defects. Medicine & Science in Sports & Exercise, 2003. 35(9): p. 1553-1563.

5. Allsworth, J., et al., The influence of stress on the menstrual cycle among newly incarcerated women. Women's Health Issues, 2007. 17(4): p. 202-209.

6. Clancy, K.B.H., et al., Endometrial thickness is not independent of luteal phase day in a rural Polish population. Anthro Sci, 2009.

7. Jasienska, G. and P.T. Ellison, Physical work causes suppression of ovarian function in women. Proceedings of the Royal Society of London Series B, 1998. 265(1408): p. 1847-1851.

8. Jasienska, G. and I. Thune, Lifestyle, hormones, and risk of breast cancer. British Medical Journal, 2001. 322: p. 586-587.

9. Strassmann, B.I., Menstrual cycling and breast cancer: An evolutionary perspective. Journal of Womens Health, 1999. 8(2): p. 193-202.

About the Author: Kate Clancy is an Assistant Professor of Anthropology at the University of Illinois, Urbana-Champaign. Her work is on evolutionary medicine, and the intersection of endometrial functioning, stress and inflammation. Kate blogs at Context and Variation and her Twitter handle is @KateClancy. She has a daughter, two cats, and has been promised a dog. These days her cycle is anywhere from twenty seven to forty days long.


The views expressed are those of the author and are not necessarily those of Scientific American.