The most fertile days of a woman's cycle are 12-16 days after the start of her last period.
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A woman is most fertile during ovulation and the week prior when there is fertile quality cervical mucus present - in fact this is when she is fertile full-stop, she cannot get pregnant any other point in her menstrual cycle. Typically ovulation occurs 14 days BEFORE menstruation, in the average 28 day cycle that would put ovulation around day 14...but if you had a 38 day cycle for example, you'd ovulate around day 24. Unless you use Fertility Awareness Method you can't know for sure when you are fertile or not, thus if you don't want to get pregnant be sure to use birth control.