Obesity and infertility

BMI Weight Status
18.5-24.9 Normal
25-29.9 Overweight
30 and Above Obese
40 and Above Morbidly Obese

What is BMI and how is it used?

Body Mass Index, or BMI, is a number calculated from a person’s weight and height, and is a fairly reliable indicator of body fatness for most people. It is used widely as a screening tool to identify possible weight problems in adults. It is often used along with other tests, such as evaluations of diet, physical activity, family history of disease, blood pressure measurements, and laboratory tests of blood sugar and cholesterol, to estimate risks of disease in a person.

How can obesity affect getting pregnant?

There is strong evidence that getting pregnant is easier with a normal body weight, which is generally found with a BMI between 20 and 25 (see above). When a person has a BMI above 25, there is a lower natural pregnancy rate and a lower chance of responding to fertility treatment. This gets worse as the BMI gets further away from the normal range.

One reason for reduced fertility rates with increasing weight is a higher chance of anovulation, meaning an egg is not released each month. Numerous studies have shown that weight loss of 5 to 10% can result in return of ovulation and improved fertility. Even with our most advanced treatments such as in vitro fertilization (IVF), pregnancy rates are reduced in a person who is obese. Obesity makes the IVF procedures of egg retrieval and embryo transfer more difficult. Even when these procedures go well, there is increasing evidence of a lower IVF pregnancy rate in obese patients.

How can obesity affect a pregnancy?

There are a number of increased risks for obese women who are pregnant. These include a higher rate of blood clot formation, high blood pressure during pregnancy, miscarriage, and stillbirth. It is also more common for obese women to deliver a premature baby. There is an increased risk of gestational diabetes, which can be difficult to manage and can lead to larger babies, more difficulty with delivery (often requiring a C-section), and some early blood sugar problems with the baby.

If an obese woman has a C-section, she is more likely to have problems with healing than a woman with a normal weight. There is also some evidence to suggest that birth defects are more common in babies born from obese women.

What are the effects of obesity on your partner?

It is also important to consider the influence that partners have on dietary and exercise habits of others in the household. Also, overweight and obese men are more likely to have lower sperm counts with lower motility (normal sperm movement), and this may impact the chance of getting pregnant naturally or having a successful fertility treatment.

For all these reasons, we think it is very important that women attempt to lose weight prior to achieving a pregnancy through infertility treatment. It would be ideal if this weight loss effort was shared by your partner, especially if your partner’s BMI is also in the overweight or obese category.

Last reviewed: 
November 2017

Interested in using our health content?