Another medical condition that could be behind an unexplained weight gain is polycystic ovary syndrome, or PCOS. A hormonal disorder that affects women, PCOS is relatively common among females in their child-bearing years, striking between 1 in 10 and 1 in 20 women in the U.S. alone [source: Women's Health].
The disease causes women to develop many small cysts on their ovaries. The cysts, in turn, wreak havoc with their hormones, causing an increase in the male hormone androgen. That hormonal imbalance results in acne, a messed-up menstrual cycle and excess body hair. It also causes women to become resistant to insulin, which regulates blood sugar, and thus may result in them getting fatter. Unfortunately, these extra pounds tend to pile on in the belly area, leaving those affected more susceptible to heart issues.
PCOS is the most common cause of female infertility. The condition may be genetic, as women with PCOS are more likely to have a mother and sister with PCOS, too. So if you're a woman whose weight has surged and some of your relatives have PCOS, get checked for it yourself [source: Women's Health].