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
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.