A: Every woman has a different "fertile" time based on her cycle length and hormone regularity. The first day of your menses (your period) is Day #1 of your cycle. It is impossible to become pregnant on Day #4 of your cycle because there's not enough time to mature an egg in 4 days. However, if you are not having a "regular" cycle each month then your ovulation (fertile time) may not be "normal." It is always best to practice safe contraception if you do not have cycles that are regular enough for "natural family planning" and if you do not want to become pregnant.