Pre ovulatory phase - Follicle, hormones, eggs, ovulating cycle

As estrogen levels begin to fall during the few days prior to the onset of menstrual flow, an increase of follicle-stimulating hormone (FSH) is triggered. This, in turn, stimulates the growth of ovarian follicles. Each follicle contains an egg. Every month, FSH initiates and promotes the growth of a group of follicles. Soon, one or two follicles assume dominance, continuing to grow, mature and produce estrogen.

The lining of your uterus begins to thicken in response to this increase in estrogen. Estrogen levels rise dramatically during the few days preceding ovulation. This is a signal that a mature follicle is ready to ovulate. The high levels of estrogen trigger the luteinizing hormone, or LH, to surge from the pituitary gland.

Over the course of a lifetime, you release about 500 eggs in their mature form. The number of eggs that are contained in the ovaries depends on your age. The highest number is actually found before you are born. While still in your mother's uterus, as a 20-week-old female fetus, you have approximately seven million eggs. At birth, the number has decreased to two million. By the time you enter puberty, you have between 300,000 and 500,000 eggs. This decline in number is the process called atresia, a natural and continuous process that is uninterruptible. Only between 400 and 500 will ripen into mature eggs during a lifetime.

During this phase, some of the ova, or eggs, in your ovaries are maturing and moving toward the surface. One of these eggs (or sometimes two) matures each month.