Risks and prevention of unintended pregnancy and STDs.(Statistical Data Included)

A fertile woman has about a 3% chance of becoming pregnant in a single act of unprotected intercourse, although the risk ranges from virtually zero to 30%, depending on when during the menstrual cycle intercourse occurs. (84) Within a year, however, a sexually active teenager who does not use a contraceptive has a 90% chance of becoming pregnant (85) (Figure 19, page 30).

Other things being equal, teenagers and women in their early 20s theoretically have a higher risk of pregnancy than older women, because the ability to conceive declines slowly after the mid-20s. (86) However, pregnancy levels may be relatively low in the first few years after puberty, before regular ovulation and sperm production are established. (87) In addition, since unmarried sexually active teenagers generally have less-frequent intercourse than older unmarried individuals, they may experience lower pregnancy rates.

The chance of acquiring a sexually transmitted infection may be substantially greater than the risk of becoming pregnant, although the likelihood varies considerably, depending on one's sex, on whether one's partner has an STD and on what the disease is. Women are at greater risk of acquiring an STD than men, because anatomical differences make many of these diseases more easily transmissible to women. (88)

* In a single act of unprotected intercourse with an infected partner, for example, a woman is twice as likely as a man to acquire gonorrhea or chlamydia (Figure 20, page 31), as well as hepatitis B (not shown).

* Some STDs are more easily transmissible than others, however. For example, in a single act of unprotected intercourse with an infected partner, a woman has a 1% risk of acquiring HIV, a 30% risk of getting genital herpes and a 50% chance of contracting gonorrhea; a man's risk of infection ranges from 1% for HIV to 30% for genital herpes.

The more partners an individual has, the higher the risk of being exposed to a sexually transmitted infection.

* Overall, younger women are more likely than older women to have more than one partner in a three-month period, because most are unmarried (89) (Figure 21, page 32).