The Apple App store has 3 apps to help a woman predict when she is fertile. A woman is fertile around the time of ovulation. These apps and the numerous websites that also offer to help determine your fertile time (e.g. fertile friend, babycenter.com) are based on these principles:
1. most women have a period about once per month
2. there are two parts to a menstrual cycle: the first part which runs from the start of bleeding until ovulation (follicular phase) and the second part which runs from ovulation until bleeding starts again (luteal phase).
3. 90% of women have a luteal phase of 14 days.
4. the follicular phase can vary
5. We count the first day of bleeding as menstrual cycle day 1
So, if someone has a menstrual cycle that is 30 days (i.e. they have a period every 30 days) then 30-14 = 16. They will likely ovulate on cycle day 16. If someone has a menstrual cycle that is 26 days then 26-14 = 12. They will likely ovulate on cycle day 12.