Contraception is avoidance of pregnancy. These include Natural Methods, Barrier methods, Intrauterine devices, Chemical methods, Surgical methods and Implants. Natural methods like periodic abstinence, lactational amenorrhea and coitus interruptus prevent meeting of sperm and ovum. Though chances are less effective these methods are popular among couples. Periodic abstinence means periodic avoidance of sex during fertility period. Female is very fertile during her ovulation days which is calculated as total number of days of Menstrual Cycle - 14 days of Secretory phase. Here the menstrual cycle is of 28 days and hence ovulation day is $28-14=14$ . i.e. 14th day is the ovulation day. This means ovum is in the oviduct and hence chances of fertilization if the sperm enters is the maximum. Thus one should avoid coitus during this period. Just to be on the safer side two or three days before and after ovulation is also considered as the period to avoid sexual intercourse.