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During sexual
intercourse, sperm are ejaculated (ejected during orgasm) deep inside
the vagina near the cervix, the small organ connecting the vagina and
the uterus. The sperm travel through the cervical mucus (thick fluid
that helps to protect the cervix and uterus from infection), into the
uterus, and up the fallopian tubes (channels connecting the ovaries to
the uterus), where fertilization normally occurs. If a sperm is able
to penetrate, or fertilize, an egg, and if other conditions are
favorable, the fertilized egg will travel from the fallopian tubes to
the uterus, where it will implant itself in the uterine lining and
fetal development will begin. A problem or obstruction at any point in
the process, however, prevents pregnancy from taking place.
Historically, men were
assumed to be fertile if they were capable of sexual intercourse. As a
partial consequence of this attitude, past research on fertility has
emphasized problems in women. Low sperm count, however, is a common
cause of infertility among couples, and the most frequent cause of
male infertility. Although only one sperm is ultimately required for
fertilization, men whose semen (fluid composed of sperm and other
secretions and produced during ejaculation) contains less than 20
million sperm per milliliter frequently have infertility problems. The
most common problem affecting male sperm levels is a varicocele,
a tangle of swollen veins surrounding the testis (the reproductive
organ that produces sperm). Surgical correction of the varicocele
restores normal fertility in about two-thirds of cases.
Other sperm problems
affect male fertility. The sperm may not be viable—that is,
structurally healthy and capable of fertilization; it may be viable
but unable to move correctly; it may contain the wrong number of
chromosomes, the packets of genetic information; or it may have been
stored too long after its formation. The vas deferens (tubes that
carry sperm from the testes to the penis) may be blocked because of a
past infection or injury. The man may not be able to ejaculate or his
ejaculation may propel the sperm backward into his bladder rather than
out through the penis. Other causes of male infertility include
insufficient hormone levels, which can be supplemented with oral
hormone treatments; prostate disease; untreated diabetes; or other
medical conditions.
Once inside the
female’s cervix, the sperm may encounter mechanical or chemical
obstacles. A muscle spasm may eject the sperm, the cervical mucus may
be too thick for the sperm to penetrate, or it may be chemically
hostile to the sperm. The fallopian tube may be blocked by scar
tissue, preventing the sperm from reaching the egg. If the sperm does
manage to reach the egg, it may not be able to penetrate the egg’s
defenses to fertilize it. A fertilized egg may become stuck in the
fallopian tube or it may not be able to implant successfully in the
uterus.
In women, one of the
most frequent causes of infertility is abnormal ovulation, or
irregular release of an egg (ovum) from the ovary. Normally one egg
will be released each month under the direction of several hormones
about midway through the
menstrual cycle. If any of these
hormones are not functioning, ovulation will occur irregularly, or
perhaps not at all. This condition accounts for about 25 percent of
cases of female infertility. Another frequent cause is blocked
fallopian tubes, accounting for about 35 percent of infertility cases.
According to the
Centers for Disease Control and Prevention (CDC), the incidence of
infertility has not changed significantly in recent years. The
perception that infertility is on the rise is probably due in part to
a broader awareness of the issue—increasing numbers of couples are
seeking medical assistance for infertility. In addition, there is a
growing tendency to delay childbearing, often until women are in their
30s. A woman reaches her peak fertility at the age of 18 or 19, with
little change until the mid-20s. Fertility then begins a slow decline
to age 35 (about 33 percent of couples in their late 30s are
infertile), a sharper decline to age 49, and a very rapid drop as a
woman nears menopause. A man’s fertility decline is not as rapid and
has no clear-cut end point, but a man of 50 is likely to be less
fertile than he was at age 25 or 30.
Physicians, usually
gynecologists for women and urologists for men, also cite other
factors that make infertility seem more common than it actually is.
These include a recent surge in the incidence of sexually transmitted
diseases, which can severely damage both male and female reproductive
systems if left untreated; and the use of birth control pills and
intrauterine devices for contraception, which can result in temporary
or permanent infertility after their use has been terminated.
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