Are You Too Thin to Conceive?

While the association between infertility and being significantly overweight is pretty clear, the relationship between being underweight and having trouble getting pregnant is not as clear. There are certain situations in which very thin women do have trouble conceiving, usually because they do not ovulate. Athletes who are training heavily may not ovulate. If they do not ovulate, they cannot get pregnant. Women with anorexia nervosa also do not ovulate normally. However, the answer to the question of whether or not you are too thin to conceive is probably no, as long as you are otherwise healthy, eating a nutritious diet, and exercising moderately.

There is a complex relationship between female hormones and hormones from the pituitary and other parts of the brain. It is relatively easy to disrupt the normal cycle of hormone release. With intense exercise, there is a disturbance in the way the brain releases hormones which can interfere with ovulation and menstruation.

Female athletes in training often stop ovulating and may have few or no periods. This means that they are also relatively low in female hormones. Marathon runners and Olympic athletes train at this kind of level. What is not completely clear is where the line is between healthy exercise and over-exercise in normal women.

Sports and physical activities that require strenuous exercise as well as a low body weight cause the most problems with the normal female menstrual cycle. This would include gymnastics, long-distance running, figure skating, and ballet. The signal that is supposed to be sent to the pituitary gland to release LH and FSH is disrupted in many women who engage in these activities. The ovaries are not stimulated, and women may stop having periods altogether.

It is believed that exercise alone does not cause disruption of the cycle. Strenuous exercise and calorie restriction both seem to be necessary for women to stop ovulating and have the cycle disruption seen in elite athletes. This is an area where there is a lot of active research into what is called the “energy drain theory.”

{ MORE: Oreos and Ovulation: How Diet Affects Fertility }

Athletes whose sports emphasize strong muscles and muscle mass, for example swimming and rowing, can also have menstrual irregularities, lack of ovulation, and infertility. These women are not calorie restricted. In these cases, women have relatively more male hormones (androgens) compared with female hormones (estrogens) in their systems. These are not thin women.

Women with anorexia nervosa lose so much weight and are so deficient in both calories and many specific nutrients that they usually do not ovulate.

In trying to determine if you are too thin to conceive, you have to know why you are thin. Are you dieting and trying constantly to lose weight? Are you exercising strenuously because you are an athlete in training or because you are trying to keep your weight low? Do you eat enough calories to maintain a normal weight? Have you lost weight because of an illness?

If you are thin and have not been having normal periods, you need to see a doctor to be evaluated. You could also have a chronic medical condition that has contributed to weight loss that may affect your fertility.

If you are a healthy woman, eating well, and having normal periods, even if you are thin, you should not have trouble getting pregnant just because of your weight.

There have been studies that show no relationship between low weight and decreased number of pregnancies or children. Women who are underweight in general have as many children as those who are normal weight.

{ MORE: To the Mom Hoping to Get Pregnant }

A preconception visit to the doctor is a great time to discuss concerns about your weight and to make sure there is nothing else wrong. You need to know that you cannot diet during pregnancy. You will need to gain weight during pregnancy as well as make sure that you get all the nutrients you and the baby will need.