A woman can become pregnant from having sexual intercourse during menstruation. In fact, recent research conducted by the United States National Institutes of Health (NIH) suggests that women have the potential to become pregnant at virtually any time during their cycle. While the chance of pregnancy is greater on certain days of a woman's cycle, it is never completely absent.

While the rhythm method may be appropriate for couples trying to become pregnant, it often fails when used to avoid pregnancy. Dr. Allen J. Wilcox of the National Institute of Environmental Health Sciences even goes so far as to say that the rhythm method does not offer a significant advantage to couples trying to conceive. According to Dr. Wilcox, couples who simply have unprotected sex on a regular basis have as much of a chance of conceiving as those who wait for fertile windows.

A woman's cycle begins on the first day of menstruation, which is counted as day one. Traditionally, days 10 to 17 are said to be a woman's most fertile period. However, this is based on the idea of a very regular cycle, lasting 28 days, with ovulation occurring on day 14. In reality, few women are this regular; only 30% of women fit this model. In addition, even women with regular cycles do not always ovulate at the same time each cycle.

A woman's fertile period lasts for about a week each month. Sperm can survive in the cervix for up to a week, and the egg is viable for 24 to 48 hours after ovulation. The problem is that ovulation is highly unpredictable. Teenagers as well as women nearing menopause - two groups that are less likely to desire pregnancy - have the most irregular menstruation cycles. Thyroid disease also contributes to irregular cycles.

In the 2005 NIH study, Wilcox and his colleagues studied the menstruation cycles of 213 women, most between the ages of 25 and 35, when a woman's cycles are most likely to be regular. They found that a woman's fertile period can extend to the day when menstruation is predicted to begin. In addition, they found that a woman can become fertile as early as day 4 of her cycle, and that 17% are fertile by day 7. Therefore, menstruation is no guarantee that a woman will not become pregnant when engaging in sexual intercourse.