II.

Causes of Infertility

A problem or obstruction at any point during conception prevents pregnancy from taking place. For about a third of infertility cases, either physicians can find no cause, or the cause can be traced to conditions in each partner that interact to cause infertility. About a third of cases can be traced to causes specifically in the male, and about a third to causes in the female.

A.

Conditions Affecting Both Partners

A number of factors that affect males and females alike can increase the risk of infertility. Perhaps the most common problem is age—the older a person is, the more difficult it is to become pregnant. Over the last 20 to 30 years there has been a trend to delay childbearing, often until women are in their 30s. A woman reaches her peak fertility at age 18 or 19, with little change until the mid-20s. As she approaches age 30, her hormone levels start to decline and her fertility also begins a slow decline, with a more rapid decline after age 35. Menopause, which occurs in the late 40s to early 50s in most women, marks the end of a woman’s natural ability to bear children. A man’s fertility decline is not as rapid and has no clear-cut end point, but a man of 50 has lower hormone levels and is likely less fertile than he was at age 25 or 30.

Genetics can also play a role in infertility. An irregular genetic makeup in one or both partners can prevent conception or result in a miscarriage, the spontaneous abortion of a fetus. Up to 60 percent of miscarriages that occur in the first three months of pregnancy result from genetic abnormalities.

Sexually transmitted infections (STIs) are a leading cause of infertility. In many cases, diseases such as gonorrhea and chlamydia may have no symptoms. If left untreated, STIs can cause extensive and irreparable damage to reproductive organs. In women, untreated STIs can cause pelvic inflammatory disease (PID), a bacterial infection that damages the uterus, fallopian tubes, and ovaries. PID is one of the primary causes of ectopic pregnancy, a life-threatening condition in which the fetus begins to develop in the fallopian tube. In men, untreated STIs can result in sterility, an inability to conceive.

In recent years fertility experts have determined that in some cases the immune system may play a role in preventing conception or interfering with embryo implantation in the uterus. Both men and women can develop an allergic reaction to sperm, causing their bodies to create antibodies that attack and kill sperm. These sperm antibodies may also bring about infertility by causing sperm to clump together, preventing them from fertilizing an egg.

B.

Male Infertility Factors

Historically men were assumed to be fertile if they were capable of sexual intercourse. As a partial consequence of this attitude, research on fertility has traditionally emphasized problems in women. More recently, however, physicians have found that the male partner is the primary cause of infertility in about 30 percent of cases. Causes of male infertility can be categorized into sperm abnormalities, structural problems, or medical disorders.

B.1.

Sperm Abnormalities

Low sperm count is the most frequent cause of male infertility. Although ultimately only one sperm is required for fertilization, men whose semen (fluid produced during ejaculation) contains less than 20 million sperm per milliliter frequently have infertility problems.

In addition to the quantity of sperm, the quality of sperm may affect male fertility. Physicians determine sperm quality according to its motility (ability to move) and its physical structure. Poor motility will prevent sperm from swimming the long distance from the woman’s vagina to the fallopian tubes to fertilize an egg. Sperm that have structural problems will also have problems penetrating an egg. Other conditions that can compromise sperm quality include genetic impairments such as damaged deoxyribonucleic acid (DNA), the genetic information critical in the development of a fertilized egg; or degradation that may result if sperm is stored too long after its formation.

B.2.

Structural Abnormalities

Some men have anatomical abnormalities that impair or prevent fertility. The most common structural problem affecting male sperm levels is a varicocele, a tangle of swollen veins surrounding the testis. Other testicular problems include torsion, in which one testis is twisted, or undescended testicles, in which the testes are located in the abdomen instead of in the scrotum, the external pouch of skin that normally holds the testes. The vas deferens (tubes that carry sperm from the testes to the penis) may be blocked because of a past infection or injury, or may be absent altogether due to a congenital abnormality. Other structural problems may prevent a man from ejaculating or cause his ejaculation to propel the sperm backward into his bladder rather than out through the penis.

B.3.

Medical Disorders

A number of medical disorders may cause male infertility. Infections such as sexually transmitted infections, prostatitis (infection of the prostate gland), and mumps contracted as an adult may lead to scarring and obstruction of the reproductive organs. Certain medications, including some prescribed to control high blood pressure (calcium channel blockers and beta blockers), ulcers (cimetidine), and depression (MAO inhibitors), can impair testicular function. Exposure to high levels of environmental toxins, including lead, mercury, and certain pesticides, may also affect male fertility. Some men have insufficient hormone levels, resulting in low sperm count or improper testicular function.

C.

Female Infertility Factors

Many factors can affect a woman’s ability to ovulate (monthly release of an egg from the ovaries), conceive, or carry a pregnancy to term. Female infertility factors are commonly grouped in two categories: structural abnormalities and hormonal imbalances.

C.1.

Structural Abnormalities

Some women are born with reproductive systems that have anatomical irregularities, or infection or injuries may damage certain reproductive organs. Blocked fallopian tubes are a frequent cause of female infertility, accounting for up to 35 percent of cases among females. Scar tissue that blocks the fallopian tubes—caused by infection, inflammation, or a condition called endometriosis—prevents eggs from meeting sperm.

Sometimes a woman is born with a malformed cervical canal. An impaired cervical canal can prevent passage of sperm from the vagina to the uterus as the sperm travel toward the fallopian tubes. If a woman is able to conceive, problems with the cervical canal can lead to miscarriage. In the uterus, noncancerous growths, such as fibroid tumors and polyps, can prevent a fertilized egg from implanting in the uterine wall.

C.2.

Hormonal Imbalance

A frequent cause of infertility in women is abnormal ovulation. Normally one egg will be released each month about midway through the menstrual cycle, under the direction of several hormones (see Menstruation). 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.

Abnormal ovulation can be caused by a number of disorders of the endocrine system, including thyroid disease, diabetes mellitus, and polycystic ovarian syndrome. Certain chemicals can affect hormonal levels and adversely affect fertility. For instance, marijuana use can shorten the menstrual cycle. Cigarette smoking reduces some types of hormone production and may deplete egg supply.

C.3.

Other Factors

A number of other factors also may contribute to female infertility. Once inside the female’s cervix, sperm may encounter obstacles. The cervical mucus (thick fluid that protects the cervix and uterus from infection) may be too thick for the sperm to penetrate, or it may be chemically hostile to the sperm. A fertilized egg may become stuck in the fallopian tube and result in an ectopic pregnancy.