Infertility is usually defined as not being able to get pregnant despite trying for one year. A broader view of infertility includes not being able to carry a pregnancy to term and have a baby. Infertility affects about 6.1 million Americans or 10 percent of the reproductive age population, according to the American Society for Reproductive Medicine.

Pregnancy is the result of a chain of events. A woman must release an egg from one of her ovaries (ovulation). The egg must travel through a fallopian tube toward her uterus (womb). A man's sperm must join with (fertilize) the egg along the way. The fertilized egg must then become attached to the inside of the uterus. While this may seem simple, in fact many things can happen to prevent pregnancy from occurring.

It is a myth that infertility is always a "woman's problem." About one-third of infertility cases are due to problems with the man (male factors), and one-third are due to problems with the woman (female factors). Other cases are due to a combination of male and female factors or to unknown causes. Infertility in men is often caused by problems with making sperm or getting the sperm to reach the egg. Problems with sperm may exist from birth or develop later in life due to illness or injury. Some men produce no sperm or produce too few sperm. Lifestyle can influence the number and quality of a man's sperm. Alcohol and drugs can temporarily reduce sperm quality. Environmental toxins, including pesticides and lead, may cause some cases of infertility in men. Problems with ovulation account for most infertility in women. Without ovulation, eggs are not available to be fertilized. Signs of problems with ovulation include irregular menstrual periods or no periods. Simple lifestyle factors – including stress, diet or athletic training – can affect a woman's hormonal balance. Much less often, a hormonal imbalance from a serious medical problem such as a pituitary gland tumor can cause ovulation problems.

Aging is also an important factor in female infertility. The ability of a woman's ovaries to produce eggs declines with age, especially after age 35. About one-third of couples where the woman is over 35 will have problems with fertility. By the time she reaches menopause, when her monthly periods stop for good, a woman can no longer produce eggs or become pregnant.

Other problems can also lead to infertility in women. If the fallopian tubes are blocked at one or both ends, the egg can't travel through the tubes into the uterus. Blocked tubes may result from pelvic inflammatory disease, endometriosis or surgery for an ectopic pregnancy.

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