A recent study conducted at the National Institute of Environmental Health Sciences in Durham NC showed that a woman's fertile window is unpredictable. It has always been thought that women ovulated about 14 days after the first day of their periods, but this study has shown otherwise. According to this study only about 30% of the women did ovulate between the 10th and 17th days of their cycles. That leaves 70% of the women to ovulate at any other time during their cycles, which did include during their periods. Because it is possible for a woman to ovulate at any time during her cycle, thinking that you can't get pregnant during your period is a myth that needs to be exterminated.
Might I add, this means the ovulation tracker only works for 30% of you.