The Science of Conception

During the first half of your menstrual cycle—the so-called follicular, or proliferative, phase—approximately 20 eggs (or ova) begin to ripen and occupy fluid-filled sacks called follicles. At the same time, the level of estrogen in your body continues to rise, causing the endometrial lining in your uterus to thicken, thereby readying it for the possible implantation of a fertilized egg and boosting the production of cervical mucus, the substance that helps the sperm to make their way to the egg.

Just prior to ovulation, rising levels of estrogen trigger a brief but intense surge of lutenizing hormone (LH) from the pituitary gland that causes the dominant follicle to rupture and release its egg. Some women experience pain in the lower abdomen as this is occurring, a sensation the Germans call "mittelschmerz" or "pain in the middle."

During the second half of your cycle—the luteal or secretory phase—the ruptured ovarian follicle (now known as the corpus luteum, or "yellow body") begins to produce progesterone, one of the key hormones that is required to sustain a pregnancy. It continues to produce progesterone until the placenta assumes this function some three months down the road.

The rising levels of progesterone cause the endometrial glands to ready the uterus for the arrival of a fertilized egg, which happens some five days later, if conception actually occurs.

When pregnancy occurs, your progesterone levels remain high. If it doesn't occur, the corpus luteum begins to regress; progesterone levels fall; and 12 to 14 days after ovulation, the uterus begins to shed the endometrial layer and your menstrual period begins.