Preparing for Ovulation
When the menstrual flow comes to a halt, the uterine lining is very thin. However, it slowly begins to thicken under the influence of estrogen, a hormone that is produced by the ovaries and released into the bloodstream. The rising estrogen level also stimulates the growth of some of the Graafian follicles that encase the immature ova. Eventually, only one of these follicles completes its growth, while the others recede. After approximately two weeks, on follicle ruptures and releases the ovum. This release is known as ovulation. At the time of ovulation, the uterine lining has thickened considerably and is close to becoming ready for a possible implantation. An ovulation occurs about 14 days before the beginning of the next menstruation. That is to say, if the ovum is not fertilized and implanted, the uterine lining will break down and will be discharged two weeks later. Thus, counting backward, in a menstrual cycle of 28 days, ovulation occurs on the 14th day; in a cycle of 35 days, it occurs on the 21st day. In other words, while the time period between ovulation and next menstruation is relatively constant, that between menstruation and next ovulation can vary considerably. Couples who practice the rhythm method of contraception have to keep this fact well in mind.