Each month, a woman's reproductive system undergoes
a complex cycle to prepare for pregnancy. Thought preparation for pregnancy actually begins a few days before her menstrual
period begins, day one of the cycle is considered the day the woman's menstrual period starts. During her period, the lining
of her uterus isshed in response to falling hormone levels from the prior month's cycle. the putuitary gland--a small endocrine
gland in the brain--releases hormones into the bloodstream. One of the hormones--follicle-stimulating hormone (FSH)--begins
to recruit (or develop) follicles in the ovaries. Follicles are round, fluid-filled compartments, each containing an egg,
and the FSH helps them grow and mature. As they grow, these follicles produce estrogen. Estrogen makes the uterine lining
thick and receptive to embryo implantation later on. The increasing estrogen level also allows glands near the cervix (the
opening into the uterus) to release thing, strechy, clear mucus. This mucus helps sperm enter the uterus through the cervix.
When the estrogen level reaches a peak, the pituitary
gland releases a hormone called luteinizing hormone (LH) into the bloodstream. The LH is releases in a surge, which signals
mprtant genetic divisions to take place in the egg; these divisions mature the egg so that it can be fertilized. The LH surge
also causes the follicle to rupture. Ovulation (the release of the egg) takes place about 34 to 36 hourse after the beginning
of the LH surge.
After the egg is releases, the follicle transforms itself
into what is called the corpus luteum, which produces and releases the hormone progesterone. Progesterone makes the uterine
lining soft, nutritive, and able to support a pregnancy. It also causes the vervical mucus to thicken and behave like a barrier
to the uterus. (This is exactly the opposite of what estrogen did around the time of ovulation!) Unless directed otherwise,
the corpus luteum will last for 9-11 days before it begins to regress (or shrink); when it regresses, the estrogen and progesterone
levels drop, the uterine lining sheds, the mestrual period begins --and the whole cycle starts again.
If, however, an egg is fertilized by a sperm, the resulting
embryo will implant into the soft uterine lining and begin producing a hormone called human chorionic gonadotropin (hCG).
The hCG signals the corpus luteum that progesterone and estrogen are still required, so taht it will not regress. THese hormones
maintain the embryo (an make the uterine lining not shed)until the placenta can take over hormone production.