It is extremely unlikely to become pregnant three days before one's period, according to WebMD. Pregnancy can only occur during ovulation, which happens around 14 days after the last period. However, every woman's menstrual cycle is different, and not all women ovulate after the same number of days.
According to WebMD, one of the best methods of determining ovulation is by monitoring the cervical mucus. Around five to six days before ovulation, the cervical mucus changes in texture and viscosity, becoming clearer, thinner and more slippery. On the day after ovulation, the mucus reverts to its original state, providing a measurable window in which a woman can become pregnant.