If you're prone to being overweight, always battling excess pounds, you probably already suspect your genes may be partly to blame. Now it turns out that one particular genetic culprit, the so-called “fat mass and obesity” gene known as FTO, may wreak greater havoc depending on when you were born.

Having a certain “twist” or variation of this gene has been found in other research to increase the risk for obesity. But according to a new study, if you were born after 1942, and especially if you were born more recently, it packs a much greater wallop than if you were born earlier.

"[The study] suggests that genetic predisposition is something that in some cases may be based on when you were born," says study leader James Niels Rosenquist, MD, PhD, instructor of psychiatry at Massachusetts General Hospital and Harvard Medical School. The study was published Dec. 29 in the Proceedings of the National Academy of Sciences.

Rosenquist and his team evaluated more than 5,000 men and women. When the study started in 1971, the participants ranged in age from their teens to their late 50s. From 1972 through 2008, the researchers collected information about height, weight and other health details on eight occasions. More than two-thirds of the men and women had their DNA evaluated to see if they had the FTO gene variant.

Rosenquist found no link between having the variant and being obese in people born before 1942. But for those born after 1942, the link was twice as great as previous research has estimated.

The later people were born, the stronger the effect of the gene variant, says Rosenquist. "The effect became statistically significant for people born in the early 1940's, and then it blew up." It became stronger and stronger as the year of birth moved up.

Differences in obesity tendency were even seen between siblings who grew up in the same environment but were born at different times.

The study did not investigate the cause of the link. But Rosenquist says the researchers suspect environmental factors contribute greatly. Changes that happened after World War II — such as an increased reliance on technology and less on physical labor, and an abundance of processed, high-calorie foods — could explain the greater impact of the gene variant,,Rosenquist says.

This was the first study of its kind to look at the impact of birth year on the gene variant.

"This study is looking at the genetic-environmental interaction from an entirely different angle," says Osama Hamdy, PhD, MD, director of the obesity clinical program at Joslin Diabetes Center, Boston. He was not involved in the study. By looking at the effect of the gene variant in people born in many different years, he says, the researchers could get a picture of how genetic predisposition may change over time, he says.

The research suggests that if you have this variant and are exposed to some of the environmental factors that favor obesity, the impact will be much greater, he says.

If today’s obesity-promoting environment doesn't change, he says, ''this could become worrisome."

Currently, more than a third of U.S. adults and 17 percent of teens and children are obese, according to estimates from the Centers for Disease Control and Prevention. 

Kathleen Doheny is a Los Angeles journalist specializing in health, behavior and fitness topics.