Infertility - Causes

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.