Correction:
Women are not most fertile during their periods, but rather 12-15 days after the first day of menses. The risk of pregnancy is not as high during a woman's pregnancy as during days 12-15. Most women ovulate around day 14 (of an average 28 day cycle where day 1 is the start of menses), but a man's sperm may survive in the vagina's hostile environment for around 2 days, making a woman's risk highest, as said, days 12-15 and not those days that she is bleeding. However, this does not mean that it is safe to have unprotected sex during that time, as some women ovulate in an unpredictable manner. Also, while it may be true that women have an increased libido during that time of the month, it isn't necessarily true for all women.
Doctor of Pharmacy Candidate