BMI & Fertility

Weight and body mass index, or BMI, can impact a woman's fertility, whether she's overweight or underweight. A man's BMI can also have an effect on fertility, particularly if he's overweight. Overweight women may also suffer from metabolic diseases that contribute to both a high BMI and problems with ovulation. Getting to a normal BMI may help couples achieve pregnancy without further treatment.

Statistics

Advanced Fertility Center of Chicago reports that women with a BMI of 29 or higher, the definition of obesity under age 35, had an ongoing pregnancy rate per in vitro fertilization cycles of 33.3 percent compared to an ongoing pregnancy rate of 54.2 percent in women of normal BMI. Between 5 and 10 percent of women have polycystic ovary syndrome, a disease that often causes a high BMI in addition to fertility problems. Women with PCOS often have problems getting pregnant because they don't ovulate. Men with a BMI in the obese range have a two-fold increase in infertility, the National Institutes of Health states. Around 12 percent of cases of infertility occur in underweight women, Wendy Chang, a doctor with Advanced Fertility Care, reports.

Causes

Many factors cause both an abnormal BMI and infertility. Overweight women with polycystic ovary syndrome have ovaries that produce excess male hormone, which suppresses ovulation. Losing weight decreases male hormone production, which may cause a woman to start ovulating. Fat cells produce excess estrogen in overweight women who don't have PCOS. Losing as little as 5 to 10 percent of body weight can improve pregnancy rates in older women, according to "Infertility Counseling: A Comprehensive Handbook for Clinicians" edited by Sharon Covington. Women with a BMI of 19 or less often stop ovulating because they produce less estrogen than normal. Overweight men may have low sperm counts.

Treatments

Losing or gaining weight helps normalize menstrual cycles. If a woman still doesn't ovulate, she'll have a better response to medications that induce ovulation once she's at a normal weight. In women who need to undergo IVF, the procedure is easier and safer in women with a lower BMI. The ovaries are easier to reach during egg retrieval through the vaginal wall in women of normal weight than overweight women.

Benefits

Losing weight not only increases pregnancy rates in overweight women, but also decreases pregnancy complications such as miscarriage, birth defects and pregnancy complications related to diabetes and high blood pressure. Being underweight also increases the risk of pregnancy complications.

Considerations

Overweight issues affect many men and women trying to conceive in the United States, with people becoming overweight at increasingly younger ages. An estimated 33 percent of children have weight problems in the United States, with the potential for menstrual irregularities and future infertility, according to KidsHealth. Losing or gaining weight is the cheapest and simplest method to possibly increase fertility without invasive procedures.