BOSTON — Exercise may be responsible for the inability of some women to become pregnant, says a study published recently pointing out that hormonal irregularities are common among women runners.
This may make them temporarily infertile even though they appear to be having normal menstrual cycles, says the study in the New England Journal of Medicine.
"A lot of women who think they're normal when they are exercising may not be," says Dr. Gary S. Skrinar, one of the researchers. "If they are trying to become pregnant and they are having problems but seem to have a normal cycle, this may be something that's worth looking at."
Other studies have found links between exercise and temporary infertility by looking for outward signs of menstrual difficulty, such as mid-cycle bleeding or delayed periods.
The new research also checked for hormonal irregularities that can disrupt fertility, and it found that these are even more common among women runners.
Women can go through normal menstrual cycles but still have subtle hormonal irregularities that prevent them from becoming pregnant.
The researchers, based at Boston University, put 28 young women on a two-month training program and found that only 14% of them had completely normal menstrual cycles while they were working out.
Before they started the experiment, all the women had normal cycles. None of them had ever done regular physical training.
They started out running four miles a day and gradually worked up to 10 miles daily. They also spent 3 1/2 hours a day in other moderately strenuous exercise, such as biking, tennis and volleyball.
The results showed that only four of the 28 women had normal periods during the two-month program.
If all had gone routinely, the women would have had 53 menstrual cycles during the exercise program. In 60% of these cycles, there were outward signs of problems, either abnormal bleeding or delayed periods. However, in 89%, there were hormonal irregularities, the study showed.
The study could not determine precisely how many of these women with hormonal disruptions would have been temporarily infertile.
Disorder in Cycles
"We are implying that there has been some disorder in their cycle, and that could surely affect their reproductive ability," says Skrinar, an exercise physiologist.
Within six months after the study, all the women had resumed normal menstrual cycles.