What causes infertility?
Infertility is caused by male and female factors: male related problems account for about 40% of infertility and female related problems account for another 40%. A combination of male and female factors represent 10% and unexpected causes account for the remaining 10% of infertlity. Male infertility can be caused by a number of different characteristics of the sperm. 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.
Fertility rates slowly decline after age 35 because of decreasing ability to ovulate (produce eggs). Women are born with all of the eggs they'll ever have. As women age, so do their eggs. Eggs of women in their late 30s and in their 40s are more likely to have chromosomal abnormalities, which helps to account for the high rate of miscarriage and Down syndrome at this age.
Adhesions (scar tissue formation) interfere with the ovaries and fallopian tubes. In endometriosis, endometrial tissue, which normally lines the uterus, implants over the ovaries and prevents the egg from entering the fallopian tube after ovulation. Pelvic inflammatory disease (PID), miscarriage and prior surgery form adhesions as well, which may also block the fallopian tubes and prevent the passage of an egg down to the uterus. PID is usually caused by a sexually transmitted disease. It can also stem from miscarriage, abortion, childbirth or an intrauterine device. Ovulatory dysfunction causes most cases of infertility. Ovulation makes eggs available for fertilization by sperm. Hormonal imbalance, such as in polycystic ovary syndrome, prevents maturation of eggs. Ovulation is absent or very irregular. Ovulatory dysfunction can be caused by hormonal abnormalities, too much exercise and other factors. Uterine fibroids may prevent implantation of the embryo or interfere with early fetal growth. Many women have fibroids, which are benign (non-cancerous) tumors. However, in some women, fibroids can lead to first trimester miscarriage.
Underlying problems include disease, such as diabetes or mumps in adult men, hormonal imbalances, endometriosis, pelvic inflammatory disease (often caused by sexually transmitted diseases, e.g., chlamydia), the abuse of alcohol and other drugs, and exposure to workplace hazards or environmental toxins. Uterine irritation or infection that sometimes accompanies IUD use can also reduce fertility. Occasionally there is a chemical or immunological incompatibility between male and female. Psychological factors are difficult to evaluate because of the stressful nature of infertility itself.
Certain environmental factors may cause men to produce a less concentrated sperm, according to a government report. Exposure to lead, other heavy metals, and pesticides has also been associated with male infertility. Many other factors, such as excessive heat exposure, microwave radiation, ultrasound, and other health hazards, are more controversial as to whether they induce infertility.
Smoking may cause infertility in both men and women. In experimental animals, nicotine has been shown to block the production of sperm and decrease the size of a man’s testicles. In women, tobacco changes the cervical mucus, thus affecting the way sperm reach the egg.
Marijuana may disrupt a woman's ovulation cycle (release of the egg). Marijuana use affects men by decreasing the sperm count and the quality of the sperm. Heroin, cocaine, and crack cocaine use induces similar effects but places the user at increased risk for PID and HIV infection associated with risky sexual behavior.
In women, the effects of alcohol are related more to severe consequences for the fetus. Nevertheless, chronic alcoholism is related to disorders in ovulation and, therefore, interferes with fertility. Alcohol use by men interferes with the synthesis of testosterone and has an impact on sperm concentration. Alcoholism may delay a man's sexual response and may cause impotence (unable to have an erection).