Maintaining a healthy weight is important for general wellness, and having a weight outside of the healthy range can be harmful to your fertility. Both underweight and overweight women are at risk of decreased fertility.
Understanding BMI
The usual way to determine if your weight falls within the “healthy” range is by calculation of your body mass index (BMI). Your BMI is calculated from your weight in kilograms (kg) divided by your height in meters squared (m2).
In general a BMI between 19 and 25 kg/m2 is considered normal for women. Overweight is defined as a BMI between 25 and 29.9 kg/m2 and obesity is defined as a BMI greater than 30 kg/m2.
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Women with low BMI (<17 kg/m2) and high BMI (>27 kg/m2) are less likely to ovulate regularly, which can lead to infertility. In overweight and obese women, anovulatory infertility is due to high insulin levels related to insulin resistance. Excess insulin can lead to high male hormone (androgen) levels, and these excess androgens can alter the normal ovarian function and ovulation.
The anovulation that occurs in infertile women with low BMI is often associated with excessive exercise or low calorie intake. These behaviors can suppress reproductive hormone production, which results in a lack of regular ovulation.
Variations do exist by race, particularly for Black women. The standard measure of abdominal obesity for women is 35 inches or more, putting them at higher risk for heart disease and diabetes. A study published in the journal Obesity found that a “high-risk” waistline for Black women was approximately 38 inches and BMI of 33, meaning that we can carry more weight before our risk for these diseases is elevated.
Improving Fertility
Weight management is critical to improving fertility in women with both high and low BMI. A weight loss of 5% to 10% can improve ovulation and pregnancy rates in infertile women who are overweight.
For infertile women with low BMI, weight gain can improve the frequency of ovulation and the chance for pregnancy.
While regular exercise is key to achieving a healthy weight, intensive exercise should be limited in women trying to conceive. As mentioned, excessive exercise in women can lead to anovulatory infertility, but even ovulatory women’s fertility can be negatively impacted by too much exercise.
Population studies have found that women who engage in more than seven hours per week of intense aerobic activity have a higher incidence of ovulatory infertility.
This negative impact may be due to low progesterone levels in the second half of the menstrual cycle or to alteration in reproductive hormones related to ovulation. The hormone leptin has recently been identified as a possible contributor to infertility in women who exercise excessively.
Leptin plays a key role in energy balance and strenuous exercise can reduce leptin levels. Low leptin levels are associated with reproductive hormone imbalance, which may cause infertility.
In women undergoing in vitro fertilization (IVF) procedures, participation in four or more hours of intense exercise per week has been associated with lower treatment success.
Men, Weight & Fertility
Less is known about the impact of weight on male fertility. There is evidence that men with increased BMI have lower reproductive hormone levels, but the effect of these lower hormone levels on sperm count and function is unclear. Some studies have found lower sperm counts in men with increased BMI, where others have found no difference compared to men with normal BMI.
Weight loss in men can improve hormone levels, but the effect of weight loss on fertility is not known.
In general, excessive exercise in males does not appear to affect fertility; although certain types of exercise (e.g. cycling and triathlon) have been associated with alterations in sperm quality.
For both men and women, a regimen of moderate exercise and dietary modification can help achieve a healthy weight and improved fertility.
