Fat, Carbs and the Science of Conception

We call the range of BMIs from 20 to 24 the fertility zone. It isn't magic—nothing is for fertility—but having a weight in that range seems to be best for getting pregnant. If you aren't in or near the zone, don't despair. Working to move your BMI in that direction by gaining or losing some weight is almost as good. Relatively small changes are often enough to have the desired effects of healthy ovulation and improved fertility. If you are too lean, gaining five or 10 pounds can sometimes be enough to restart ovulation and menstrual periods. If you are overweight, losing 5 percent to 10 percent of your current weight is often enough to improve ovulation.

Being at a healthy weight or aiming toward one is great for ovulatory function and your chances of getting pregnant. The "side effects" aren't so bad, either. Working to achieve a healthy weight can improve your sensitivity to insulin, your cholesterol, your blood pressure and your kidney function. It can give you more energy and make you look and feel better.

While dietary and lifestyle contributions to fertility and infertility in men have received short shrift, weight is one area in which there has been some research. A few small studies indicate that overweight men aren't as fertile as their healthy-weight counterparts. Excess weight can lower testosterone levels, throw off the ratio of testosterone to estrogen (men make some estrogen, just as women make some testosterone) and hinder the production of sperm cells that are good swimmers. A study published in 2006 of more than 2,000 American farmers and their wives showed that as BMI went up, fertility declined. In men, the connection between increasing weight and decreasing fertility can't yet be classified as rock solid. But it is good enough to warrant action, mainly because from a health perspective there aren't any downsides to losing weight if you are overweight. We can't define a fertility zone for weight in men, nor can anyone else. In lieu of that, we can say to men who are carrying too many pounds that shedding some could be good for fertility and will be good for overall health.

The Importance of Exercise
Baby, we were born to run. That isn't just the tagline of Bruce Springsteen's anthem to young love and leavin' town. It's also a perfect motto for getting pregnant and for living a long, healthy life. Inactivity deprives muscles of the constant push and pull they need to stay healthy. It also saps their ability to respond to insulin and to efficiently absorb blood sugar. When that leads to too much blood sugar and insulin in the bloodstream, it endangers ovulation, conception and pregnancy. Physical activity and exercise are recommended and even prescribed for almost everyone—except women who are having trouble getting pregnant. Forty-year-old findings that too much exercise can turn off menstruation and ovulation make some women shy away from exercise and nudge some doctors to recommend avoiding exercise altogether, at least temporarily. That's clearly the right approach for women who exercise hard for many hours a week and who are extremely lean. But taking it easy isn't likely to help women who aren't active or those whose weights are normal or above where they should be. In other words, the vast majority of women.

Some exciting results from the Nurses' Health Study and a handful of small studies show that exercise can be a boon for fertility. These important findings are establishing a vital link between activity and getting pregnant. Much as we would like to offer a single prescription for conception-boosting exercise, however, we can't. Some women need more exercise than others, for their weight or moods, and others are active just because they enjoy it. Some who need to be active aren't, while a small number of others may be too active.

Instead of focusing on an absolute number, try aiming for the fertility zone. This is a range of exercise that offers the biggest window of opportunity for fertility. Being in the fertility zone means you aren't overdoing or underdoing exercise. For most women, this means getting at least 30 minutes of exercise every day. But if you are carrying more pounds than is considered healthy for your frame (i.e., a BMI above 25), you may need to exercise for an hour or more. If you are quite lean (i.e., your BMI is 19 or below), aim for the middle of the exercise window for a few months. Keep in mind that the fertility zone is an ideal, not an absolute. Hospital delivery rooms are full of women who rarely, or never, exercise. Not everyone is so lucky. If you are having trouble getting pregnant, then maybe the zone is the right place for you.