Most people know that a woman's fertility decreases as she gets older, but even during her most fertile (and otherwise healthy) years, lifestyle choices and external factors can affect a woman's chances of having a healthy baby. 

Obesity

Carrying around extra pounds can affect hormone production and make it more difficult for a woman to get pregnant, says William Schlaff, MD, chairman of the department of obstetrics and gynecology at Jefferson Medical College, Thomas Jefferson University, Philadelphia. "The more weight a woman gains over her healthy weight, the more she tends to experience decreased ovarian function," he says. And a 2009 study published in Fertility and Sterility found that women who were obese at age 18 were more likely to develop polycystic ovary syndrome (PCOS) and have problems with infertility. PCOS is the most common hormonal disorder among women of reproductive age, and the leading cause of infertility.

Being too thin

Just as excess body fat can impact fertility, so can not having enough of it. That may be because women who have very low BMIs are deficient in leptin, the hormone that controls hunger and feelings of satiety. Having low leptin contributes to the absence of menstrual periods, according to a 2009 Harvard University study.

Household chemicals

Exposure to pollutants, pesticides, and industrial compounds can decrease a couple's ability to have children by up to 29%, according to a 2013 study published inEnvironmental Health Perspectives. Additionally, a 2015 Washington University study found that 15 common chemicals were associated with early menopause. These chemicals include nine PCBs (which have been banned since 1979 but still exist in older products), three pesticides, two forms of plastics called phthalates (often found in personal care items and beauty products like perfumes and nail polishes), and the toxin furan, a byproduct of industrial combustion.