BOSTON -- In Tuesday's medical news, NewsCenter 5's Heather Unruh reported on arthritis relief, sleepless nights and the United States' collective weight gain.

Arthritis Relief

Many women find relief from the painful symptoms of rheumatoid arthritis when they're pregnant, and now researchers said they may know why.

A new study showed that arthritic symptoms eased when moms-to-be had higher levels of their baby's DNA in their bloodstream.

The exact reason why isn't known, but arthritis sufferers often report that their pain subsides during pregnancy.

Sleepless Nights

Women who have the toughest time battling the hot flashes of menopause are also the most likely to have sleepless nights, according to new research.

Stanford University researchers surveyed 1,000 middle-aged women and found that 80 percent of them had regular hot flashes. The women also shared symptoms of chronic insomnia.

Chronic insomnia is defined as poor quality sleep, as well as trouble falling and staying asleep, that continues for at least six months.

Weight Gain Theories

There are several reasons why Americans are collectively growing more obese, according to new research in the International Journal of Obesity.

Air conditioning, researchers said, can contribute to weight gain because our bodies aren't burning as many calories regulating temperature. Other theories include: there are fewer people smoking -- kicking the habit tends to make people eat more; more Americans are not getting enough sleep; some medications used for diabetes and mental illness might lead to weight gain; and studies have linked having an older mother to heavier body weight.