A woman is least fertile immediately before menstruation.
More Info: To become pregnant, male sperm needs to reach and fertilize the female egg. Fertility follows a predictable cycle. During the first stage of the cycle, there is a build up of tissue in preparation for pregnancy. Then around day 14, an egg is released from a woman's ovary into the fallopian tube. This process is known as ovulation. Typically, sperm can survive inside a woman's body for approximately 72 hours. The absolute most fertile period is immediately before ovulation. If there is already sperm inside of a woman at the time of ovulation, then she has a high likelihood of getting pregnant.
Why Are Women Least Fertile During Menstruation?
Once a woman's body determines that there won't be a pregnancy in a particular month, it begins to shed the lining of the uterus that has been built up over the course of the month. This is often called the "period", or menstruation. It is during this time that a woman is the least fertile. Although the exact time of ovulation can be tough to pin down, the likelihood of pregnancy decreases dramatically at a time when her body is deliberately shedding the necessary tissues needed for pregnancy.
Can I Count On Menstruation as Birth Control?
Of course, this process isn't perfect. Many women have irregular cycles that can be tough to time, and sometimes sperm can be quite tenacious and survive many times longer than the average survival rate of 72 hours. It is possible to get pregnant anytime you have sexual intercourse. However, if your cycle is regular, you can reduce the odds of getting pregnant if you have sex in the day or two immediately preceding your period.