Every month, a woman’s uterus prepares for pregnancy, except when she is already pregnant, on birth control hormones, or perhaps nursing a baby. The whole cycle usually takes about four weeks, as long as a lunar cycle, though anywhere from three to five weeks may be within normal range for an individual woman, and sometimes the cycle is less regular than that. Most months, the woman does not get pregnant, so the nest her uterus has prepared for the baby bleeds out. That is called menstruation.
The menstrual cycle is said to begin on the first day of a menstrual period. For convenience, since a cycle really has no beginning, let us say that it begins at the end of the previous period.
The uterine lining begins to build up, preparing a space for a developing fetus. After about a week to a week and a half, it is ready. One of her ovaries (a woman has two, one on either side of her body) releases an egg cell. This is called ovulation. The egg travels down the fallopian tube connecting the ovary to the uterus, a journey lasting several days.
If the woman has sex with a man during this time or a few days before, and he releases sperm cells into her body, the egg may encounter a sperm cell and be fertilized. If it does not, the egg passes out of the body after a few days in the uterus. It passes unnoticed, too small to be seen.