Ovulation is a part of the menstrual cycle when the ovary releases a ripe egg, or ovum. Inside the ovary are hundreds of thousands of follicles. Each follicle is a hollow ball of cells with an immature egg in the center. The typical 28 day menstrual cycle begins on the first day of menstrual bleeding. During the first seven days of the cycle, a few follicles begin to grow at the same time. These maturing follicles secrete estrogen hormone into the bloodstream to prepare the lining of the uterus for pregnancy. Around day seven, all of the follicles stop growing and begin to generate, except for one. This dominant follicle continues to grow and nourishes the developing egg inside it. Around day 12, the follicle secretes a large amount of estrogen into the bloodstream. When the estrogen reaches the hypothalamus and pituitary gland in the brain, the anterior, or front part of the pituitary gland, releases a huge surge of luteinizing hormone into the bloodstream. Around day 14, luteinizing hormone causes the follicle to undergo a sudden growth spurt. Right before ovulation, the egg detaches from the inside of the follicle. The bulging follicle releases chemicals, causing one of the two fallopian tubes to move in closer and surround the follicle. The follicle swells until it bursts open, ejecting the egg and fluid from the follicle into the abdominal cavity. In response the fimbriae, tiny projections at the end of the fallopian tube, sweep across the ovulation site and pick up the egg. Microscopic cilia on the fimbriae surface, transport the egg to the entrance of the fallopian tube. Inside the walls of the fallopian tube, muscular contractions gently push the egg towards the uterus. After ovulation, the egg lives for 12 to 24 hours, so it must be fertilized by a sperm from the male during this time for a woman to become pregnant. If it's not fertilized, the egg dissolves away and is shed along with the uterine lining during menstruation.