Pills, implants, and injections all have one goal: to prevent your reproductive system from producing a mature egg. They do this by tricking the system into skipping a key step in the interlocking cycle of hormone production that triggers the egg's release from the ovary. The deception works like this:

Under ordinary circumstances, the brain's hypothalamus produces GnRH (gonadotropin-releasing hormone). This prompts the pituitary gland to release FSH (follicle stimulating hormone) which travels to the ovaries through the bloodstream and causes a follicle to grow. The development of the follicle produces estrogen, which after about 10 days reaches high enough levels to trip off a surge of LH (luteinizing hormone) from the pituitary gland. The ovarian follicle releases a mature egg into the fallopian tube about 24 hours after this surge of LH, and the empty follicle becomes known as the corpus luteum. The cells of the corpus luteum produce progesterone and estrogen, which together stimulate the uterine lining to thicken with blood in preparation for nurturing a fertilized egg. Once the corpus luteum wanes and the lining is left with no hormonal support, it sloughs off during your monthly period. The low levels of estrogen and progesterone also signal the hypothalamus to start the process over again.

Since oral contraceptives (OCs) and Lunelle Injection provide a steady level of both progestin (a substitute for progesterone) and estrogen every day, and Norplant implants and Depo-Provera provide steady daily levels of progestin, there is no signal to the hypothalamus to release GnRH and therefore no signal to the pituitary gland to produce FSH and LH. Because FSH stimulates the ovaries to grow egg follicles, and LH triggers ovulation, their absence causes the ovary to be relatively dormant, and no egg is produced to a point where it could be released. Hormonal contraception locks the system into the same late phase of the cycle on a continuous basis, perpetually skipping the all-important release of GnRH.

How Hormonal Methods Short-Circuit the Reproductive Cycle Hormonal contraceptives work by damping down the two key hormones that trigger ovulation. Follicle stimulating hormone (FSH), the substance that coaxes an egg towards maturity, is the first to be suppressed. Luteinizing hormone (LH), which ordinarily triggers release of the egg at mid-cycle, is also held down. Production of both these substances usually starts when the body senses a shortage of two other hormones: progesterone and estrogen, both produced in the ovaries. Hormonal contraceptives supply just enough of these substances to prevent start-up of the FSH/LH production cycle. Constant levels of estrogen and progesterone thus produce constant levels of FSH and LH, and the eggs remain dormant.

Suppression of ovulation is the main mode by which OCs, Depo-Provera, and Lunelle prevent pregnancy; the implant system causes ovulation suppression about 50 percent of the time. However, throughout each pill cycle, and continuously with Norplant implants and Depo-Provera, the mucous covering the cervix—the site where sperm enters the uterus—stays thick and sticky, making it very difficult for sperm to get through. This gooey impediment also acts on the sperm cell itself. It prevents fertilization by interfering with chemical changes inside the sperm that allow it to penetrate an egg's outer coating.

Even if ovulation and fertilization do take place, hormonal methods provide another measure of protection: changes to the uterine lining. Normally, estrogen initiates the thickening of the lining of the uterus in the first part of the cycle, while progesterone kicks in later to help the lining mature. Since both hormones are present throughout the pill cycle, and progestin is supplied continuously by implants and the shot, the usual hormonal variations are masked and the lining rarely has a chance to develop enough to nurture a fertilized egg.

All the hormonal methods currently available to us offer many benefits, including protection from cancer. However, they aren't 100 percent effective, and they aren't right for all women. To correct this, scientists are busy developing new forms of hormonal contraception which may be easier to use and may suit more women. These methods include biodegradable implants, pellets the size of a grain of rice, and a new product called the vaginal ring. Like a diaphragm, this device is removable. But unlike barrier contraceptives, it releases steady levels of progestins to prevent pregnancy.

Even without these new approaches, the array of choices at your disposal is varied and wide. Before you decide on a method take time to weigh the benefits and risks of all the forms of hormonal contraception available today. The following overview provides the basic information you'll need, but be sure to discuss any questions with your physician. Together you can find the approach that's optimal for you personally.

The Pill: Pros and Cons To help you decide whether oral contraception is for you, here's a brief overview of its pluses and minuses: Advantages: Highly effective Does not interrupt sex Safe for most women Protects against ovarian and endometrial cancer Decreases menstrual cramps and pain Reduces menstrual blood flow, thereby reducing anemia Is easily reversible Is easy to use and discontinue Has been well researched Disadvantages: Offers no protection from sexually transmitted diseases Can be expensive Produces rare but dangerous complications May cause mood changes May give rise to nuisance side effects such as headaches, weight gain, and breakthrough bleeding Must be taken every day