Earlier research on exercise and fertility
Research on infertility in the late 1980s focused on the impact of various lifestyle behaviors and emotions on fertility. Some mind/body programs for infertility were founded in 1987 based on some preliminary research which showed that teaching infertile women various stress-reducing strategies resulted in higher pregnancy rates. As part of the program, women received recommendations about lifestyle changes, based on the current available research.
One of the recommendations was to decrease the frequency and intensity of their exercise regimens. There hadn't been any research on the impact of exercise on fertility in humans, but there was some animal research that showed that animals of differing species had lower pregnancy rates when they exercised more.
Psychological connections between exercise and pre-pregnancy
There is an element of intuitive sense here -- if you go out for a run, you might be running to reduce stress, or to allow yourself dessert that evening, but, instinctively, your body thinks you are being chased by a bear (or just escaping danger). So if you are running three to five times a week -- although you are doing it to work off the stress from work or so your pants fit -- your body actually perceives that you are in danger several times per week, and the only reason you are still alive is your ability to outrun that danger. So if your body thinks you are escaping danger so regularly, it might follow that your body may not allow you to get pregnant, which would make you larger, awkward, slower and far more vulnerable to that danger.
What experts had to say about it
Early on, there wasn't enough research to support the idea of intense exercise having a negative impact on the ability to conceive. Patients of infertility specialists would even be encouraged to exercise more to reduce the stress of treatment. Then in 2006, a study came out in the journal Obstetrics and Gynecology. The conclusion of the study was that vigorous exercise was linked with lower pregnancy rates from IVF treatment. The study stated, "Regular exercise before in vitro fertilization may negatively affect outcomes, especially in women who exercised four or more hours per week for one to nine years and those who participated in cardiovascular exercise." Even at that time, most infertility specialists didn't believe the results, and they criticized the design of the study.
The evidence at last
Fast forward to March 16, 2012. The American Society of Reproductive Medicine (ASRM) sent out a bulletin to all 10,000 members, highlighting a new study that indicated that vigorous exercise delayed pregnancy in normal weight women. In this study, Danish researchers followed 3628 women who were planning on becoming pregnant. They completed questionnaires every two months for 12 months or until they achieved pregnancy. For normal weight women, the more vigorous their exercise, the longer it took them to get pregnant. Moderate exercise was associated with the shortest time to conception. There was no relationship for obese women however.
If you are a healthy weight and would like to get pregnant, talk to your doctor about taking your exercise routine down a notch. Or two. We still don't know if the same relationship exists for women who already have infertility and are in treatment, but decreasing one's exercise intensity is such an easy thing to try. If your doctor approves, it could be worth a try for you!