Did you know that when a baby girl is born, she already has about 1,000,000 ovarian follicles? Each ovarian follicle contains a hollow ball of cells with an immature egg in the center. During childhood, approximately half of ovarian follicles are absorbed by the body. By the time a girl reaches puberty and her menstrual cycle begins, only about 400,000 ovarian follicles are left to develop into mature eggs.
Although only one egg usually fully matures during ovulation, somewhere between ten and 20 follicles begin the process of maturation monthly. The excess ovarian follicles are reabsorbed before ovulation occurs.
The process of ovulation is started and controlled by a drop in the hormone estrogen (mostly etradiol) to a low level. When the drop in estrogen levels occurs the hypothalamus (a part of the brain) is signaled to increase its secretion of gonadotropin-releasing hormone (GnRH) thus sending a message to the pituitary gland to increase its secretion of follicle-stimulating hormone (FSH). The increase in FSH causes the growth of ten to 20 of the ovarian follicles.
Estrogen is secreted by some of the cells in the follicle. Just before ovulation occurs, the follicle that contains the maturing egg eases toward the surface of the ovary. Once the matured egg reaches the ovarian surface, ovulation occurs when the follicle and the ovarian surface open allowing the egg to drift out of the ovary.
Additionally, progesterone is also produced by the cells in the ovarian follicles shortly before ovulation occurs. After ovulation, if pregnancy has not occurred, the empty follicle is called the corpus luteum and it is reabsorbed into the body. If pregnancy does occur the corpus luteum produces hormones that help to maintain the pregnancy.
After the egg is released from the ovary it travels to the oviducts (the funnel-shaped ends of the fallopian tubes) where it begins its long journey of several days into the uterus. The mature egg is moved along on its journey through the fallopian tubes by wavelike muscle contractions in the fallopian tube. The inner lining of each fallopian tube contains cilia which are constantly beating microscopic hairs; these cilia are what helps move the sperm towards the egg if a woman has had unprotected sexual intercourse. Conception (the fertilization of an egg with sperm) most often occurs in the part of the tubes that is nearest the ovary. Five to six days are required for the fertilized egg to reach the uterus.