Risk Factors

In the U.S., an estimated 10.2% of women between the ages of 15 - 44, or about 6.1 million women, have impaired fertility, and the incidence is increasing. About 25% of women experience some period of infertility during their reproductive years.

Age

As a woman ages, her chances for fertility decline. Infertility in older women appears to be mostly due to a higher risk for chromosomal abnormalities that occur in her eggs as they age. Older women are also more likely to have health problems that may interfere with fertility. If fertilization occurs, older, healthy women can usually successfully bear a fetus to term, although they have a higher risk for miscarriage. Using population studies, experts have come up with estimated odds for pregnancy at different ages, given no fertility intervention. A 2002 analysis of pregnancy rates based on conception on the day of ovulation suggested that women between ages 19 - 26 have twice the pregnancy rates as those between 35 - 39.

Weight Factors and Excessive Exercise

Although most of a woman's estrogen is manufactured in her ovaries, 30% is produced in fat cells by a process that transforms circulating adrenal male hormones into estrogen. Because a normal hormonal balance is essential for the process of conception, it is not surprising that extreme weight levels, either high or low, can contribute to infertility.

Being Overweight. Being overweight or obese (fat levels that are 10 - 15% above normal) can contribute to infertility in various ways. Obesity is highly associated with polycystic ovarian syndrome (PCOS), which is cause of infertility in some cases. In one 2003 study, overweight women without PCOS were classified in one of five grades, depending on the severity of the obesity. The risk for irregular or absent periods increased two-fold by each increase in grade. In this group, amenorrhea (absent periods) was also highly associated with type 2 diabetes and blood sugar abnormalities.

Being Underweight. Body fat levels 10 - 15% below normal can completely shut down the reproductive process. Women at risk include:

• Women with eating disorders, such as anorexia or bulimia.
• Women on very low-calorie or restrictive diets are at risk, especially if their periods are irregular.
• Strict vegetarians might have difficulties if they lack important nutrients, such as vitamin B12, zinc, iron, and folic acid.
• Marathon runners, dancers, and others who exercise very intensely. (Lower body fat contributes to menstrual irregularities in competitive athletes, but other mechanisms are also involved.)

Environmental Risks

Exposure to environmental hazards (herbicides, pesticides, industrial solvents) may affect fertility. Estrogen-like hormone-disrupting chemicals are of particular concern for infertility in men and for effects on offspring of women.

Phthalates, chemicals used to soften plastics, are under particular scrutiny for their ability to disrupt hormones. Specific phthalates of special concern include dibutyl phthalate (DBP) and others found in many products, including cosmetics and clay products sold to children (Fimo, Sculpey). Animals exposed to phthalates have significantly impaired sperm count and abnormalities in reproductive structures, such as the testes. In addition, there is some concern that exposure in pregnant women may affect the offspring.

Stress and Fertility

Neurotransmitters (chemical messengers) act in the hypothalamus gland, which controls both reproductive and stress hormones. Severely elevated levels of stress hormone can, in fact, shut down menstruation. Whether stress has any significant effect on fertility or fertility treatments is unclear. One 2005 study found that psychological stress does not affect the success or failure of in vitro fertilization.

Click the icon to see an image of the hypothalamus.