According to new study from Boston University measuring the effect of various exercise levels on fertility rates in Danish women and published in Fertility & Sterility, there was evidence for a dose-response relationship between increasing vigorous physical activity and delayed time to pregnancy in all subgroups of women with the exception of overweight and obese women. Moderate physical activity was associated with a small increase in fertility regardless of BMI. These findings indicate that physical activity of any type might improve fertility among overweight and obese women, a subgroup at higher risk of infertility. On the other hand, lean women who substitute vigorous with moderate physical activity may also improve their fertility.
A previous Harvard study was defining vigorous expercise as > 3 hr/week. This one is > or = 5 hr/week so the breakpoint is around 4-5 hr. The bottom line is that a 30 minute walk most days of the week, the current national guideline, is also a good level for fertility.