A study has shown that up to 70 percent of women -- particularly perimenopausal women in their 40s -- are fertile during the time of month formerly thought to be safe.
Up to now, ob-gyns believed the vast majority of pregnancies occur during a fertility window which spans days 10 through 17 of a woman's menstrual cycle.
But the study -- conducted by the National Institute of Environmental Health Sciences -- reports that "there are few days of the menstrual cycle during which some women are not potentially fertile."
Researchers studied 696 cycles from 213 women to determine how effective the rhythm method is. Allen J. Wilcox, M.D., concluded in the British Medical Journal, "Women should be advised that the timing of their fertile window can be highly unpredictable, even if their cycles are usually regular."
What's more, it is even possible for a woman to conceive on the day when her period is supposed to start.
By the time women enter their 40s, many hope that their fertility has decreased to the point where they can get off the Pill and use a more natural method of contraception. The rhythm method, also known as natural family planning, has long been thought to provide adequate protection in this age group.
Given the results of this study, however, they may want to think again.
