Does my weight affect my fertility?

With New Year’s resolutions still going strong, your weight in relation to your fertility may be on your mind. Research shows that a body mass index (BMI) between 20-25 is ideal to conceive, and losing or gaining weight to fall into that range can significantly improve your fertility outcomes. Most research uses the BMI as a function of weight and height – if you’d like to figure out yours, here’s an easy calculator to do so. Since BMI is not a perfect measure (e.g., it doesn’t distinguish between fat and muscle), we recommend you also discuss your weight with your doctor. Here’s the more detailed high-quality research on weight and fertility:

One of the largest studies on weight and natural conception, a Danish study of over 64,000 pregnant women, found that those with a BMI over 25 and particularly over 30 had a much lower likelihood of conceiving. In another Scandinavian country, researchers studied over 26,000 Norwegian men and showed that they were much more likely to have fertility issues if they had a BMI below 20 or above 23, particularly once it climbed above 27.

Analogous to the findings on natural conception, being over- or underweight also negatively impacts the outcomes of fertility treatments: Overweight women tend to respond less to follicle-stimulating hormones and ovulation-inducing medication. Consequently, they tend to have fewer eggs that can be fertilized, and fertilization rates as well as the quality of embryos tends to be lower. This in turn affects pregnancy rates and increases the risk of miscarriages. For underweight women, a BMI <19 is often associated with irregular or lacking periods and hormone imbalances, and can impact the ability to carry a pregnancy.

That’s why doctors sometimes recommend delaying fertility treatment in order to gradually lose or gain weight and often run additional blood tests to check for imbalances, for example of insulin or of thyroid hormones. The research on diets that boost fertility suggests that you avoid fast food and instead embrace fruits, vegetables, legumes, fish, whole grains, and foods high in omega 3, such as nuts. This “Mediterranean diet” has also been linked to higher sperm count and sperm quality in men.

For more tips on how to support your fertility in 2020, read the 12 tips in our previous post and, as always, talk to your doctor.


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