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. The release of the egg from the ovary is called ovulation. Some women know when they're ovulating, because at mid-cycle they have some pain typically a dull ache on either side of the lower abdomen lasting a few hours. 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 estrogen and progesterone help build up the uterine lining to prepare for pregnancy. Because the length of menstrual cycles varies, many women 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.