Monday, September 19, 2011

Do really skinny women have fewer children?

Does extreme thinness cause a woman to be less fertile? I looked at women ages 40-59 who participated in the MIDUS Study. I limited the sample to people with at least a 4-year degree. Here are the mean number of children by body mass index (BMI) classification (sample size = 369):

Mean number of children

BMI under 20 1.45
20-29 2.10
30-39 2.08

A BMI under 20 is extreme in this sample: this group is only 3 percent of the total sample. And you can see that they have fewer kids.


  1. Yes, my wife is considered by most people to be fairly thin, at 115-120 pounds and about 5'3, and that's still around a BMI of 21. To get below 20, you need to be considerably lighter than that. I do know a couple of women in that category, and they did in fact have a hard time reproducing.

  2. Anonymous5:15 PM

    Is it adjusted for age as well? As women age they put on weight, and they also have (more) children. You'll have to confirm that the three BMI cohorts have the same average age to draw a meaningful conclusion.

  3. The women are ages 40-59.

  4. Causation could easily go the other way, though. It would be better to have BMI at age 20 and # of kids at age 40 (even then causation could go the other way, but it would be better).

    And BMI<20 = Skeletor

  5. Yeah, I suspect a relatively large proportion of women under BMI 20 either do not menstruate, menstruate irregularly or have anovulatory cycles. Under BMI 15 and fertility all but stops... which makes perfect evolutionary sense: Starving mothers don't tend to raise successful offspring.

  6. Skinny people often have comparable or worse life outcomes than obese people in these kinds of datasets. The male fertility probably looks similar.

  7. Anonymous11:50 AM

    White women have fewer children than non-white women in the US, and they also tend to have a lower BMI. (or be less overweight, to be blunt)

    Hard to say if there's any cause and effect going on here though. Do they have a lower BMI because they had fewer (or no) children, or did they have fewer (or no) children because they have a low BMI? Or is neither of these the case?

    Here is the data on obesity (BMI > 30)

    Non-Hispanic Black Women: 49.6 percent
    Hispanic Women: 43 percent
    Non-Hispanic White Women: 33 percent

    Breaking the data on number of children down by race as well as BMI might be interesting.

  8. Anonymous5:17 PM

    Isn't the conventional wisdom that women tend to put on weight after they have kids? That's as likely an explanation for the GSS data. (I don't doubt that both factors are valid to some degree.)


  9. Anonymous7:09 PM

    I am 5'5 and can't stay above 20 BMI if I do any exercise at all. I had to get to 126 lbs in order to conceive.

    I only gained 15 lbs while pregnant and was back under 20 BMI about 3 weeks after delivery.

    I only have two kids and spent about 12 years total using no birth control but did no fertility testing, treatments etc.

    "And BMI<20 = Skeletor"

    Generally true, but I have a very small frame. I don't look bony. Lots of really young women are very low BMI. At age 16 I was like 50th %tile for weight. However, I never gained any weight after that. So, now I am like the 4th %tile by just staying the same height and weight.

  10. Anonymous10:54 AM

    I know a sixty-two inch tall woman who weighs ninety-seven pounds. That's a BMI of 17.7. She has three children.

    Of course, she's Japanese. And my sense is that the "normal" BMI varies by race. And perhaps by country as well - the typical European-American is more massive than the typical European, and the typical African-American is more massive than the typical African.

  11. Ron, these are interesting results, but why did you limit the sample to women who had 4 yr + degrees?
    I'm wondering if the results would be different with a broader sampling?

  12. More educated women are thinner and have fewer kids, so I wanted to adjust for that.

  13. Thanks! I figured you must have a reason for narrowing the sample, but I couldn't figure out why.


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