More than 15 percent of women may have difficulty becoming pregnant at some point during their lifetime. The term infertility is used when pregnancy does not occur after at least one year of trying. Since infertility affects about 7.3 million couples in the U.S., interest is growing in how diet and lifestyle might affect fertility.

About a third of infertility cases are attributed to the male partner, a third to the female and a third to a combination of both partners (or no cause is found). Male infertility is usually attributed to an insufficient healthy sperm count. Women with infertility can trace their issues to a number of different causes: problems with the structure of the uterus, blocked fallopian tubes or, most often, failure to release healthy eggs.

Several diet and lifestyle issues may also have an effect. Here's a list of the most common:

Weight

A woman's weight can affect her fertility. In fact, weight issues account for about 12 percent of all infertility, according to the American Society for Reproductive Medicine. Too little body fat reduces estrogen levels so much that the reproductive cycle starts to shut down. Too much body fat raises estrogen levels, acting like birth control pills to suppress ovulation (egg release). Obesity is even linked with decreased success of fertility treatments.

And women aren't the only ones whose fertility is threatened by excess weight. Several studies link excess weight in men with decreased semen and increased production of abnormal sperm. Some researchers suggest that eating plenty of vegetables and fruits, which provide antioxidants to prevent damage to sperm, may protect men's fertility. But overall, research results have been very mixed.

Polycystic ovarian syndrome

PCOS is an endocrine disorder and the most common reason for infertility due to lack of ovulation. In women with the disorder, insulin and reproductive hormones are at abnormal levels, and egg production and menstrual cycles often halt. Besides infertility, PCOS is linked with greater risk of developing diabetes, heart disease and some types of cancer. Fortunately, a modest weight loss of about five percent is often enough to normalize hormones and improve fertility. Regular physical activity seems important as well, and a diet that includes more dietary fiber and whole grains may also help.

Anemia

A lack of healthy red blood cells is linked with female infertility, caused by both lack of iron or inadequate vitamin B-12 or folate. Although more research is needed in this area, women can avoid most anemias through a healthy diet. To start, make sure to get enough folate by eating plenty of vegetables, beans and grains–both whole grains and enriched grains. If you don't eat animal products (including dairy) make sure to get adequate vitamin B-12 from fortified foods or supplements.

Diet

Overall balanced eating habits may also help with infertility. One large study involving over 17,000 nurses reported that women who follow healthy eating practices experienced up to 66 percent lower risk of infertility due to ovulation problems. The women who benefitted had a diet high in healthy monounsaturated fat (found in olive oil and nuts, for example), low in trans fat and rich in vegetable-based protein and whole-grain foods. These eating habits may help by promoting normal insulin levels and by providing a variety of antioxidants that could promote and protect pregnancy.

Tobacco

Tobacco use threatens both male and female fertility. It makes a woman's eggs more prone to genetic abnormalities and has been linked with abnormal sperm, decreased sperm and impaired sperm movement. Nicotine and other chemicals can also affect ovulation by decreasing estrogen.

One note of caution: We must be careful when interpreting results from many of these studies, since observational data can only speak to associations between various diet and lifestyle choices and fertility, not causation.