Risk factors
Many of the risk factors for both male and female infertility are the same. These include:

Age is the strongest predictor of female fertility. After about age 32, a woman's fertility potential declines. A woman does not renew her oocytes (eggs). There is no one special point when fertility declines — it's a gradual transition.

Chromosomal abnormalities. Infertility in older women may be due to a higher risk of chromosomal abnormalities that occur in the eggs as they age. Older women are also more likely to have health problems that may interfere with fertility. The risk of miscarriage also increases with a woman's age. A gradual decline in fertility is possible in men older than 35.

Tobacco smoking. Women who smoke tobacco may reduce their chances of becoming pregnant and the possible benefit of fertility treatment. Miscarriages are more frequent in women who smoke.
Alcohol. There's no certain level of safe alcohol use during conception or pregnancy.

Body mass. Extremes in body mass — either too high (body mass index, or BMI, of greater than 25.0) or too low (BMI of lower than 20.0) — may affect ovulation and increase the risk of infertility.

Being overweight. Among American women, infertility often is due to a sedentary lifestyle and being overweight. Being underweight. Women at risk include those with eating disorders, such as anorexia nervosa or bulimia, and women on a very low-calorie or restrictive diet. Strict vegetarians also may experience infertility problems due to a lack of important nutrients such as vitamin B-12, zinc, iron and folic acid. Marathon runners, dancers and others who exercise very intensely are more prone to menstrual irregularities and infertility.