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A resting follicle which contains an egg, a group of primordial follicles will begin to grow for reasons still unknown. This growth must begin in the month prior to an egg's intended ovulation. The follicles grow through the stages of primary follicle, secondary follicle, tertiary follicle, and finally to be called a preovulatory Graafian follicle, measuring about 1.5 centimeters in diameter.
The number of growing follicles in each month's group is inversely related to a woman's age; that is, the younger she is, the more primordial follicles will begin to mature, etc.
Estrogen is secreted by the growing follicles themselves, causing a reduction in the amount of FSH production. This appears to sort out the strongest eggs from the growing group, so that by the end of Week 1, there is usually just one follicle left -- the dominant follicle, approximately 8 to 10 millimeters in size. It is literally a small cyst.
This completes the follicular phase of ovulation and begins the luteal phase. We are now at approximately CD 14:
High levels of estrogen in the blood signal the release of a surge of LH -- this same surge is the one detected by home ovulation predictor kits (OPK's). LH causes the follicle cells to start making use of progesterone, which is being secreted by the ovary.
Generally, ovulation takes place approximately 36 hours after the start of the LH surge, 20 hours from the time of peak LH levels in the blood.
After the follicle opens and releases its oocyte, it becomes a new structure, the corpus luteum. The corpus luteum is responsible for producing high levels of progesterone, necessary to sustain a pregnancy.
Progesterone levels tend to peak in the bloodstream on or around CD 21, and without being signalled by hCG (hormone emitted by a pregnancy), it will fall rapidly and another follicular phase begins. If, on the other hand, conception occurs, the corpus luteum will continue producing progesterone until the placenta takes over.
The luteal phase tends to be around 11 to 16 days in length.
The entire process of ovulation takes around 24 to 35 days to complete.
© Tracy Morris
