April 10, 2013

Menstrual “cycling”: what’s so “regular” about it?

What's a "normal" menstrual cycle? Does every woman ovulate every cycle? Why would the timing of menstrual cycle events vary?

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Menstrual cycle

Menstrual cycle

In this post, I’m discussing something most of us take for granted—menstruation. You probably know the basics, and some of you are probably wondering, why is she wasting her time on this? Get to the myths already! But some of the most interesting myths surrounding female reproductive functioning are rooted in a poor understanding of the events of the menstrual cycle and how much their timing really varies from one woman to the next (inter-woman variation).

Events of the menstrual cycle in a nutshell

There are only two phases of a menstrual cycle: the follicular (or proliferative) phase and the luteal (or secretory) phase, separated by ovulation. During the follicular phase, a follicle, which surrounds the oocyte, or egg, matures. The follicle itself is a cluster of specialized cells that produces estrogen and progesterone (the ovarian hormones), which in turn directly regulate the growth of the endometrium (the lining of the uterus, or womb) while also regulating the growth and eventual release of the oocyte itself. Estrogen is the most important hormone during this time, as it is responsible for the growth of the endometrium and will eventually trigger the release of a second hormone, LH, from the anterior pituitary gland (a chick-pea—sized gland that sits on the bottom of your brain). This LH surge triggers the release of the oocyte from the ovary—and this is ovulation. Ovulation marks the end of the follicular phase and the beginning of the luteal phase. The follicle that was surrounding the oocyte becomes a specialized structure called the corpus luteum, and has a very important role in the second phase of the cycle.

For most of the follicular phase, progesterone levels are fairly low, but during the luteal phase, they rise dramatically because the corpus luteum releases relatively large quantities of progesterone. Progesterone stimulates the activity of the endometrium, preparing it for possible implantation if fertilization (the joining of oocyte and sperm) has occurred. If implantation does not occur, then the luteal phase ends when menstruation begins: the lining of the endometrium is shed, menstrual bleeding begins, and a new follicular phase is initiated.

The timing of these events is where things get tricky

So now we know what happens in The Menstrual Cycle. Or do we? How universal is this pattern? As you can see in the above graph, the menstrual cycle is typically conceived of as a cyclical phenomenon reoccurring every 28 days, like clockwork. Each of the phases is assumed to last 14 days, with ovulation occurring on day 14, without fail. I don’t have room to go into the reasons why our society is obsessed with machine-like understandings of the human body (but, see Marks, 2001, or Emily Martin’s work for more on this idea), but this kind of simplistic, mechanistic understanding of how human bodies function leads to erroneous assumptions about what is normal/healthy vs. abnormal/unhealthy. For example, there are those who think that if a woman doesn’t have her period every 28 days, she must be sick in some way—especially hormonally.

However, many studies have demonstrated that there is actually a great deal of variation in the timing of ovulation and in the overall length of the menstrual cycle. For example, Chiazze and colleagues (1968) found that 95% of women’s cycles were between 15 and 45 days long. In another, Ferhing and colleagues (2006) showed that 95% of women’s cycles were between 22 and 36 days. In other words, most women do not have 28-day cycles! Perhaps even more surprising is that individual women experience highly variable menstrual cycle lengths over their lifetimes. After analyzing data from 628 women, Gorrindo et al. (2007) reported that only about 28% of women had “stable” lifetime menstrual histories—in other words, more than two-thirds of women were “irregular” cyclers. If this many women can be classified as “irregular,” it’s probably time to start reconsidering what it means to be “regular”! In fact, menstrual cycle length varies between and within women so much that the World Health Organization suggested we stop using the word “cycle” and refer to menstrual segments instead, to avoid the associations that “cycle” brings to mind (Snowden and Christian, 1983).

If menstrual cycle length varies so much within and between women, it makes sense that the phases of the cycles vary greatly, too. There is evidence (Ferhing et al., 2006) that the follicular phase varies more than the luteal phase, which makes estimating the timing of ovulation very tricky—we basically have a moving target! Furthermore, ovulation itself is not guaranteed—a number of things (including energetic stressors) can result in reduced ovarian activity. It has been estimated that even in healthy Western women, only about 80% of cycles can be assumed to be ovulatory (Metcalf et al., 1983). I’d like to stress that that 80% is in healthy women who are not undergoing any particular stressors—in women experiencing energetic or psychological stresses above and beyond their normal circumstances, ovulation may be even less likely to occur. Evolutionary anthropologists like Virginia Vitzthum have suggested that this reduction in ovarian activity during times of acute stress may be a natural, adaptive response to harsh conditions, in which resources would be better spent on non-reproductive functioning.

So, any assumptions about if/when a woman ovulates or when she “should” be in either phase of the menstrual cycle have to be made with extreme caution. Even better, assumptions should be avoided in lieu of actual testing for the occurrence of ovulation!

For review of each of the studies mentioned in this post, see Harris and Vitzthum, 2013.

In my next blog, I’ll talk about PMS, premenstural syndrome–is it “real”?