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Infertility is an inability to conceive after 12 months of unprotected sexual intercourse in an effort to become pregnant. It refers to a state in which a couple has a diminished capacity to conceive. It should not be confused with sterility, which is a physical inability to become pregnant.

About 6.1 million couples in the United States, or 10 percent of all couples of childbearing age, have difficulty conceiving, according to the American Society for Reproductive Medicine (ASRM). About one-third of infertility cases can be attributed to males, another one-third to females and the remaining one-third to both members of a couple, or to causes that go unexplained. The latter make up roughly 20 percent of infertility cases.

Infertility occurs because the human reproductive process is so intricate that even a minor disruption may be enough to derail conception. Each month, hormones produced in the woman’s pituitary gland signals the ovaries to prepare an egg for ovulation. These hormones are called follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

Women are most fertile during this period of ovulation, which occurs around the 14^th day of the menstrual cycle. However, the exact time of ovulation varies among women because of normal differences in the length of the menstrual cycle. After a woman ovulates, the egg is captured by the fallopian tube and begins its passage to the uterus. For fertilization to occur naturally, the sperm and egg must unite in the fallopian tube. The greatest odds of fertilization occur in the first 12 hours. Sperm can fertilize the egg for up to 72 hours after ejaculation. If fertilized, the egg moves and implants into the uterus two to four days later.

For conception to occur, hundreds of variables must develop in just the right way. When no fertility problems are present, the average couple between the ages of 29 and 33 has about a 20 to 25 percent chance of becoming pregnant during any given menstrual cycle, according to the National Women’s Health Resource Center.

Infertility results when something disrupts this process and prevents conception. This can be due to problems with the hormones, the egg or sperm or with transport, penetration of the egg by sperm and fertilization. It can be due to medical or mechanical issues. There are many treatments that can help couples overcome obstacles to conception. Although not always successful, these methods often result in fertilization and the eventual birth of a healthy child.