The length of a menstrual period can vary significantly from woman to woman. In general, your most fertile time is about 14 days before the estimated start of your next period.
The first day of the menstrual cycle is the when the bright red bleeding begins. Your uterus sheds its lining starting on Day 1 if you're not pregnant.
The Hormone Chain
While menstruation is under way, your body gears up for ovulation, which is the release of an egg. The ovarian follicles produce estrogen, which must build up to a certain level before an egg is triggered to release. It takes several days for estrogen levels to rise.
After an egg has matured, a hormone called luteinizing hormone (LH) triggers the release of the egg. The egg is released between 12 and 36 hours of the hormone surge.
For pregnancy to occur, you must have intercourse during the days leading up to ovulation, on the day the egg is released or, at the latest, the following day. Most women probably do not enjoy sex when their menstrual flow is at its heaviest.
Possibility of Pregnancy on Day 3 of Cycle
Getting pregnant on Day 3 of your cycle is highly improbable; however, if you have irregular periods or a short cycle, you could ovulate right after your period. Having intercourse during ovulation can result in pregnancy.