SUSAN P. PHILLIPS
Contraception is the prevention of conception or impregnation in a woman. Interference with any aspect of fertilization or implantation of a fertilized egg can prevent pregnancy. Mechanical or chemical barriers may be used to block the production of egg or sperm, alter their function, obstruct fertilization (the meeting of egg and sperm to form an embryo), or alter the uterine environment to prevent implantation. Abortion, described elsewhere, can also be considered a contraceptive method. Information about the way a contraceptive works, its failure rate, risks, noncontraceptive benefits, costs, and availability are required to make an informed choice. There is no best method because risks and benefits vary from person to person.
The average menstrual cycle is twenty-eight days. By convention, day one refers to the first day of bleeding or menses. During the follicular phase (day one until ovulation), the shedding of the lining of the uterus (menses) is followed by increasing pituitary production of the follicle- stimulating hormone (FSH) and the luteinizing hormone (LH). FSH stimulates the development of several follicles in the ovary, one of which assumes dominance by days five to seven. The follicles in turn produce an increasing amount of estrogen, which initiates regrowth of the lining of