Depends. A menstrual cycle is usually 28 days, with ovulation occurring mid cycle around day 14. You can get pregnant around this time. Sperm can stay active in the body for 3-5 days. If you are not ovulating, you cannot get pregnant. However, everyone's cycle is different. If you want to get pregnant there are ways to help track ovulation. If not, it is best to use birth control all the time.
14 & 0. During the actual menstrual cycle, a woman cannot conceive. About 12-16 days later is when ovulation occurs, and fertility is at its height. Relying on the cycle to plan children is called the "rhythm method." it's best to rely on barrier protection and oral contraceptives to help plan for a pregnancy. Using all three is the most effective method of planning or preventing a pregnancy.