Workload and reduced fecundity — try not to work too hard, ladies

Here’s a vaguely misogynistic study for you.  This article, “Women who work or lift a lot may struggle to get pregnant,” discusses the findings from a recent paper in Occupational and Environmental Medicine.  The authors surveyed women trying to conceive in the Nurses’ Health Study 3, a large cohort of predominantly Caucasian nurses.  Their covariates of interest were how many hours per week women worked and how often they lifted more than 25 lbs in a day; the primary outcome was time to conception.  The authors concluded,

Working more than 40 hours a week was linked with taking 20 percent longer to get pregnant compared to women who worked 21 to 40 hours.

Moving or lifting at least 25-pound loads several times a day was also tied to delayed pregnancy, extending the time to conception by about 50 percent.

The unstated interpretation is that women’s bodies can’t handle working a full day or lifting any weight, so women at reproductive age should think twice about what they do for a living.

Here’s the original paper.

One potential issue is that the study is cross-sectional and the authors didn’t actually follow the women from the time of first interview until they got pregnant.  Instead, they used one survey to ask how long the women had been trying to conceive, then used a survival analysis method to estimate the time to pregnancy based on the self-reported times.  This method of analysis is biased: women who had no trouble conceiving are underrepresented in the sample and women who have taken a long time to get pregnant are overrepresented.  Furthermore, we don’t know the true outcomes for these women, only the ones estimated by a parametric model.

My biggest issue with this study is that they attach any meaning to their findings at all, saying that working more has a “detrimental impact on female nurses’ ability to get pregnant”.  They use duration of pregnancy attempt “as a surrogate for fecundity”.  Fecundity implies some biological ability to reproduce.  However, using time to conception as a proxy for fecundity relies on the assumption that everyone is trying equally hard to get pregnant.  If that were the case, then any variation in time to conception would be due to fecundity.  This isn’t something they checked or measured, and differences in women’s ideas of what “trying to get pregnant” means are probably what’s actually driving the trend the authors reported.

The Reuters article quoted someone sensible:

“If this effect is real, it is likely due to the fact that these women are having less frequent intercourse due to their work demands,” Lynch, who wasn’t involved in the study, said by email.

Nobody needed to do a study to figure that out.  Anyway, we could come up with all sorts of other plausible explanations for why women who work more are having less frequent intercourse.  If they redid this study on a cohort of women working in tech, I’m sure they’d find a similar relationship between number of hours worked and time to conception.  The point is, working more hours or picking up 25 lb boxes probably has no effect on anyone’s biological capacity to reproduce.  The authors are making a mountain out of a molehill.

I routinely lift 100 lbs over my head, so I guess I’ll really be screwed when I want to have a baby.

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2 thoughts on “Workload and reduced fecundity — try not to work too hard, ladies

  1. This research study sounds as though it is suffering from confirmation bias.

    Anyone who works a full-time job, regardless of gender, will have less time for sexual intercourse. If your job requires you to lift things regularly, you will also be more physically exhausted when you do return home.

  2. Ahh love this post! It can be so easy to look at a paper’s abstract, note the results and never really check the method. The more time I spent in school, the more aware I become of this pitfall but I can still find it an easy trap to fall into at times. Thanks for shedding light on this one.

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