HUMAN BIOLOGY

316 Chapter 16

preventing pregnancy

refrain from intercourse during the woman’s fertile period,

starting a few days before ovulation and ending a few

days after. Her fertile period is identified and tracked by

keeping records of the length of her menstrual cycles and

sometimes by examining her cervical secre tions. A woman

who uses this method may also take her temperature each

morning when she wakes up, because core body tempera-

ture rises by one-half to one degree just after ovulation

(Figure 16.11). The method is not very reliable (Table 16.3).

Ovulation can be irregular, and it can be easy to miscalcu-

late. Also, sperm already in the vaginal tract may survive

until ovulation.

Withdrawal, removing the penis from the vagina before

ejaculation, also is not very effective because fluid released

from the penis before ejaculation may contain sperm.

Douching, or rinsing out the vagina with a chemical right

after intercourse, is next to useless. It takes less than 90 sec-

onds for sperm to move past the cervix into the uterus.

surgery and barrier methods

are the most effective options

Controlling fertility by surgery is less chancy but is usually

an irreversible step. In vasectomy, a physician makes a tiny

incision in a man’s scrotum, then severs and ties off each

vas deferens (Figure 16.12A). Afterward, sperm can’t leave

the testes and so can’t be present in the man’s semen. A

vasectomy does not change a man’s sex hormones or sex

drive. An alternative is the Vasclip, a device about the size

of a rice grain that simply closes off the vas deferens.

In tubal ligation, a woman’s oviducts are cauterized

or cut and tied off (Figure 16.12B), so sperm cannot reach

ovulated oocytes.

Spermicides kill sperm. They are packaged inside an

applicator and placed in a woman’s vagina just before inter-

course. Neither is reliable unless used with another device,

such as a diaphragm or condom.

A diaphragm is a flexible, dome-shaped device that is

positioned over the cervix before intercourse. It must be

fitted by a doctor, used with foam or jelly, and inserted

correctly with each use. A cervical cap is a similar but

smaller device and can be left in place for up to 3 days.

The contraceptive sponge is a disposable disk that contains a

spermicide and covers the cervix. After being wetted, it is

inserted up to 24 hours before intercourse. No prescription

or special fitting is required.

The intrauterine device, or IUD, is a plastic or metal device

that is placed into the uterus, where it hampers implanta-

tion of a fertilized egg. Available by prescription, IUDs

have been associated with a variety of complications and

should be discussed fully with a physician.

Condoms are thin, tight-fitting sheaths of latex or animal

skin worn over the penis during intercourse. Good brands

may be as much as 95 percent effective when used with

n    People who choose to control whether their sexual activity

produces a child have a variety of options.

The most effective method of birth control is complete

abstinence—no sexual intercourse whatsoever. A modified

form of abstinence is the rhythm method, also called the “fer-

tility awareness” or sympto-thermal method. The idea is to

16.7

Method Mechanism of Action Pregnancy Rate*

Abstinence Avoid intercourse entirely 0% per year

Rhythm method Avoid intercourse 25% per year

when female is fertile

Withdrawal End intercourse before 27% per year

male ejaculates

Vasectomy Cut or close off male’s <1% per year

vasa deferentia

Tubal ligation Cut or close off <1% per year

female’s oviducts

Condom Enclose penis, block 15% per year

sperm entry to vagina

Diaphragm, Cover cervix, block 16% per year

cervical cap sperm entry to uterus

Spermicides Kill sperm 29% per year

Intrauterine device Prevent sperm entry <1% per year

to uterus or prevent

implantation

Oral contraceptives Prevent ovulation <1% per year

Hormone patches, Prevent ovulation <1% per year

implants, or

injections

Emergency Prevent ovulation 15–25% per use**

contraception pill

Table 16.3 Common Methods of Contraception

Figure 16.11 Ovulation coincides with a slight rise in

body-core temperature.

© iStockphoto.com/Ever.

*Percent of users who get pregnant despite consistent, correct use

**Not meant for regular use

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