If you're wondering why infertility happens, know that you're not alone. Here are some of the common causes of infertility in women and men.

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If you're wondering whether you or your partner is infertile, know that you're not alone: About one in ten couples experience infertility.

The good news is that the majority of infertility cases can be treated, whether by addressing the cause or by methods such as artificial insemination or assisted reproductive methods.

Infertility is pretty much an equal-opportunity condition, meaning that; both women and men can be the source of infertility. About one third of cases are attributable solely to the woman, and about one third just to the man. (The remainder are because of both the man and woman, and in some cases, the cause is unexplained.)

For women, there are several conditions and factors that cause most of the cases. Let's start with age — it's the single most important determining factor for a successful and healthy pregnancy. When a woman is around age 35, her egg quality and quantity decline significantly.

"To put it into perspective, a 25-year-old healthy woman may have a 30 percent chance of pregnancy trying the good old-fashioned way, a 38-year-old woman has a 10 percent chance, and a 42-year-old woman has only a 1 percent chance," says Liyun Li, M.D., a fertility specialist at the Pacific Fertility Center in San Francisco.

Aside from age, one of the most common causes of female infertility is infrequent ovulation (the release of a mature egg from the ovary). "This is often the result of hormonal problems or conditions such as polycystic ovarian syndrome, in which a woman produces excess male hormones and does not ovulate regularly," says Kathleen M. Brennan, M.D., a reproductive endocrinologist with the UCLA Department of Obstetrics and Gynecology in Los Angeles. Your levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) also need to be within certain parameters for ovulation to happen regularly.

Fallopian tube blockage is another possible culprit. Blockage is typically the result of scarring, from a pelvic infection (such as an STD like chlamydia), a previous ectopic pregnancy (when a fertilized egg becomes implanted in the fallopian tube instead of the uterus), or surgeries such as appendectomy or removal of ovarian cysts (a prior abortion rarely affects fertility). Some women are also prone to developing growths called fibroids or polyps in the uterus; this may block fallopian tubes or lead to infertility and miscarriages.

Both decreased ovarian function and blocked fallopian tubes can be caused by endometriosis, a condition in which the lining of the uterus grows into other places in the abdomen, Dr. Li explains. Endometriosis can cause scarring and can produce substances that interfere with conception. (If you have painful periods — one of the main signs of endometriosis — talk to your doctor.)

The best thing women can do to safeguard her fertility? "Live a healthy lifestyle," Dr. Brennan says. Maintaining a healthy weight and not drinking too much alcohol can up your chances of getting pregnant (anything more than one drink a day is excessive for women). Smoking is a big risk factor — it can damage your fallopian tubes, age your eggs prematurely, and increase your risk of miscarriage and ectopic pregnancy.

As for the male causes of infertility, your partner may not be producing enough sperm, or his sperm may be slow moving or not moving at all. He might have a duct problem that keeps sperm from being released into ejaculate, or he may be suffering from enlarged blood vessels in the testicles that can increase the testicular temperature and hinder sperm production.

A healthy lifestyle is just as important for men as for women, Dr. Brennan says. Excess alcohol (more than two drinks a day for men), smoking (cigarettes and marijuana), and even stress can affect sperm production and function. A man's age plays a role, though not as much as a woman's age does — testosterone, sperm quality, and sperm quantity decline throughout the years — as does having a health condition such as diabetes, cancer, and kidney diseases.

When should you consult a doctor?

If you are under 35 and you and your partner have been trying for a year, talk to your doctor about undergoing fertility testing to find the cause, Dr. Li says. If you are older than 35 and have been trying for at least 6 months, experts recommend that you consult a doctor. Of course, anytime you have concerns — regardless of your age — you should talk to your doctor.