During menstruation, levels of estrogen, made by the ovaries, start to rise and make the lining of the uterus grow and thicken. It is encased in a sac called the Graafian follicle, which continues to produce estrogen as the egg grows. At about day 14 of a typical 28-day cycle, the sac bursts and the egg leaves the ovary, traveling through one of the fallopian tubes to the uterus. The release of the egg from the ovary is called ovulation. The medical word for this is mittelschmerz, from the German, meaning middle pain.
Some women also have very light bleeding, or spotting, during ovulation.
The rising levels of both estrogen and progesterone help build up the uterine lining to prepare for pregnancy. Because the length of menstrual cycles vary, many woman ovulate earlier or later than day 14. It's even possible for a woman to ovulate while she still has her period if that month's cycle is very short. The placenta then makes hormones and provides nourishment from the mother to the baby. If an egg is not fertilized that month and the woman doesn't get pregnant, the corpus luteum stops making hormones and gets reabsorbed in the ovary.