Inside your ovaries each month, a group of eggs begins to grow in small, fluid-filled sacs called follicles. Ovulation occurs when one of the eggs erupts from the follicle, usually about two weeks before your period starts. After the egg leaves the follicle, your body releases a hormone that helps thicken the lining of your uterus to prepare it for the egg. The egg moves through the fallopian tube, where conception takes place. The egg stays in the fallopian tube for about 24 hours waiting for fertilization by a single sperm.