Young women with a weight problem often say the weight started creeping up when they had their first child and they found they had less time to exercise. When researchers added up all factors, though, they found that the fact that a woman is married and has a baby has more influence on weight gain than being physically active.

That is the key message from the Australian Longitudinal Study on Women's Health, a 10-year study from the School of Human Movement Studies at the University of Queensland. The findings appear online and in the February issue of the American Journal of Preventive Medicine.

"The weight gain appeared to start when they married, then worsened when they had their first child," says lead author Wendy J. Brown. "There was no effect on the rate of weight gain of having a second baby."

From 1996 to 2006, researchers periodically surveyed a randomly selected group of about 6,500 Australian women ages 18 to 23 at study's start.

"Women with no partner and no baby averaged 11 pounds over 10 years. With a partner and no baby they gained about 15 pounds, and if they had a partner and a baby they gained 20 pounds," Brown says.

"The so-called energy-balance variables like eating too much and moving too little had an effect, but the estimates of weight gain are adjusted for differences in these factors," she says.

Brown says that young women ages 18 to 33 are gaining weight at a higher rate than their mothers' generation. "If it continues, this generation will end up with more health problems later in life. It is important to understand the causes of this weight gain."

A U.S. expert offers a different explanation for the weight-gain gap.

"Eleven pounds gained by single women is interesting. As far as I know, weight gain is not a physiologic consequence of normal aging, but more a reflection of cheap, widely available food, and less physical activity," says Dr. Julie Fagan, a women's health specialist with University of Wisconsin Hospital and Clinics, in Madison, Wis.

"Weight control issues for married women with kids include less time to prepare nutritious meals and more reliance on fast food, takeout, and processed food. Women may overeat due to mindless ingestion of comfort food," Fagan says. "This is particularly true during the newborn stage. Sleep deprivation can lead to eating to try and fuel the brain to stay awake. We know that people who sleep more tend to weigh less."

Single women "may gain less because they are still dating, want to attract a mate, don't necessarily eat three large meals daily, and they have more time for exercise," Fagan suggests.