Every month in one of a woman's ovaries, several immature eggs begin to develop. One of the cells in this group will be chosen to mature completely, suppressing growth of the others. This “dominant follicle” will later rupture and release the egg from the ovary- known as ovulation. Ovulation generally occurs about two weeks before a woman's next menstrual period is due to begin.
After the egg is released, the remaining ruptured follicle in the ovary forms a structure called the Corpus Luteum which is responsible for producing the hormones progesterone and estrogen. Progesterone has the important job of thickening and preparing the lining of the uterus (endometrium) for the embryo to implant and develop in. Once the egg has been released from the ovary it moves down the woman's fallopian tube and this is where fertilization may occur. In general, the ovum can be fertilized for approximately 24 hours after ovulation. If the egg is not fertilized the Corpus Luteum will degenerate, causing hormonal changes and the break-down of the endometrium which results in menstrual bleeding. The Female Reproductive Cycle A key element of reproduction is the woman's menstrual cycle. This cycle can be divided into three phases: Follicular Phase During this phase, a hormone known as FSH stimulates a follicle to secrete estrogen. Estrogen widens the cervix and changes the consistency of the cervical mucus to ensure better accessibility for sperm. During the follicular phase, one follicle will become dominant and the egg within it will begin to mature. Ovulatory Phase In the next phase, LH level increases dramatically and causes the ovary to release a ripened egg. In a typical 28 day cycle, this usually occurs 14 days after the first day of a woman's period. Luteal Phase During this final phase, the ovarian follicle that nurtured the egg prior to ovulation becomes known as the corpus luteum. The corpus luteum secretes a hormone known as progesterone that prepares the uterine lining to nourish a fertilized egg. If fertilization does not occur during this cycle, the corpus luteum ceases to function around the 26th day and the uterine lining breaks down and sheds. The shedding of the uterine lining is known as a menstrual period.
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