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What is the “biological clock,” anyway?

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June 27, 2016   |   In the News, Studies Say, The Real Deal

What is the “biological clock,” anyway?

A recent article in the Guardian—a piece with many good and important points, overall—claimed that “the idea of the biological clock is a recent invention.”

While it may be true that the phrase “biological clock” was first used as a metaphor for women’s reproductive systems in the 1970s, the actual biological clock—the fact that every organism is, more or less, on a timeline that’s set by nature—is not a new phenomenon. It applies to every single species on the planet, and refers to a broad range of body systems, such as waking, sleeping, and aging—not just fertility.

It’s important to separate the social construction of the “biological clock” (and the negative stereotypes of women associated with that idea) from the scientific realities of life. The fact is that the biological clock includes the natural process of aging which, according to wide medical consensus, affects a woman’s fertility dramatically (more so, research has shown, than it does a man’s).

Many, many studies are quite clear on this conclusion. A report published by the CDC found that the pregnancy rate decreased by about 53% between ages 25–29 and ages 35–39. Also often cited is the historical study mentioned in the Guardian piece, which reviewed birth records from France between 1670–1829 to determine that pregnancy rates for women over 35 are only about 66%.

The author of the Guardian piece—and ASRM itself—note that, since a couple’s sexual activity also often decreases with age, it can be difficult to separate the effects of sexual behavior on pregnancy from the effects of age. It can also be difficult, when examining historical records, to account for women who may have tried to avoid unwanted pregnancies with techniques such as the rhythm method. That’s why another study examined the pregnancy rates of healthy, fertile women trying to conceive through insemination with donor sperm. This controlled for the effect of the male partner, desire for pregnancy, and frequency of sexual activity, leaving the focus specifically on the age of the woman. The results of that study confirmed that pregnancy rates decreased progressively with the increasing age of the woman. “The cumulative pregnancy rate observed across up to 12 insemination cycles was 74% for women younger than 31 years,” the study found, “decreased to 62% for women aged 31–35 years, and to 54% for women older than 35 years.”

A similar pattern of declining fertility can also be seen in studies of IVF success rates. According to the CDC, “the percentage of IVF (in vitro fertilization) cycle starts that resulted in live births was 41.5% in women younger than 35 years, 31.9% in women aged 35-37 years, 22.1% in women aged 38-40 years, 12.4% in women aged 41-42 years, 5% in women aged 43-44 years, and 1% for women older than 44 years.” Taking into consideration the results of these many studies, the American Society for Reproductive Medicine estimates that, generally, a woman’s fertility “decreases gradually but significantly beginning approximately at age 32 and decreases more rapidly after age 37 years.”

This is why the Guardian article’s dismissal of the “biological clock” should be taken with a grain of salt. “Would it not be more straightforward simply to admit,” the author suggests, “that both men and women have bodies that age—and that most humans share basic desires for affection, intimacy, and respect?” This is a nice statement, but it doesn’t give women all of the information they deserve to have about their own bodies.

Yes, the stereotype of the weepy woman fretting over her “ticking” biological clock is patronizing and sexist. Not every woman’s life revolves around having children—nor should it—and women’s decisions shouldn’t be based on the pressure of their biologies. But being aware of the way the biological clock affects our bodies is crucial to making the best choices we can. We’re not doing women any favors by leaving them uninformed.


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