The field has done an extensive job looking at how body fat correlates to fertility and, just as importantly, how losing weight improves a person’s fertility. Before we delve into the data we probably need to get centered on how body fat is measured in most studies. We’ll constantly refer back to Body Mass Index (BMI), which is a function of a person’s weight (in kilograms) divided by their height (in meters squared). While BMI has its imperfections (like the inability to distinguish between fat and muscle) it’s often an excellent proxy for body fat in most people and, unlike just using weight, takes into account a person’s height.
Generally speaking a “normal” female BMI is 18 - 25 but because being too light can impact a person’s ability to conceive, the ideal range for hopeful parents is 20 - 25.
Generally speaking, being overweight or underweight can impact the quality of a woman’s eggs and potentially the quality of her uterine lining and the ability to carry a pregnancy.
In this large Dutch study below, you’ll notice that as BMIs climb above 25, the odds a woman will be unable to conceive also rise in a pretty steady fashion. What we like about this study is investigators corrected for the male partner’s BMI because often women with high BMIs are coupled with men who have high BMIs and, as you’ll see, that factor independently raises the risk of infertility.
Men who have an abnormally high or low BMI encounter bigger challenges. Below is data on over 20,000 Norwegian men that pitted their BMI against their relative likelihood of being unable to conceive. The analysis compares everyone against those with a normal BMI (in this case 20 - 22.5), smartly corrects for factors like the female partner’s BMI, and reminds us that not only is being too heavy a problem but so too is being too light.
When a couple must resort to IVF, there is clearly a tight inverse correlation between a woman’s rising BMI levels and diminishing odds that any given IVF cycle will work. While this is most pronounced in women with PCOS, it’s also true for women who have no fertility issue (who are just doing treatment for male factor infertility), and the IVF population at large.