Children of mothers who smoked and were overweight during pregnancy are at most risk of becoming obese later in life, a new study finds.

Elevated risks already associated with smoking during pregnancy include premature birth, miscarriages, certain birth defects and infant death.

Researchers in the latest study were able to predict a greater risk for a obesity by examining the child’s mother’s behavior around their birth, according the study done at the University of Montreal.

"Although behavior is extremely hard to change and is also influenced by a complex tangle of influencing factors in the environment, I hope these findings will help improve the social and medical services we offer to mothers and infants," said lead author Laura Pryor, a PhD candidate at the university's Department of Social and Preventive Medicine.

Using the Quebec Longitudinal Study of Child Development that ran from 1998 to 2006, Pryor and her team analyzed data on 1,957 children, whose height and weight measurements had been taken yearly, from the age of 5 months to eight-years-old, enabling them to calculate the children’s body mass index (BMI).

The authors explained that BMI is “calculated as weight in kilograms divided by height in meters squared,” and said that the researchers identified three trajectory groups, which include, children with low but stable BMI, children with moderate BMI, and children whose BMI was elevated and rising, called high-rising BMI.

"We discovered the trajectories of all three groups were similar until the children were about two and a half," Pryor said in the study published by the journal Pediatric and Adolescent Medicine.

"Around that point the BMIs of the high-rising group of children began to take off. By the time these children moved into middle childhood, more than 50 per cent of them were obese according to international criteria." Researchers found that the mothers' weight around the time they gave birth and whether the mothers smoked had an influence on the child’s weight.

Children that had mothers who were “overweight or who smoked during pregnancy were significantly more likely to be in the high-rising group,” the authors wrote. These two factors were found to be much more important than the other criteria that were studied, such as the child's birth weight.

Researchers found that both factors were much more important than other criteria studied, such as the child’s birth weight.

As more research will be required to determine how these early-life factors have an influence on childhood obesity, the risk factors found in this study represent increased probabilities of becoming overweight, but the authors explained that they’re not direct causes and the researchers want to broaden their study and follow these children until they reach adolescence.

"Our research adds to the growing evidence that the perinatal environment has an important influence on later obesity," Pryor said. "This points to the need for early interventions with at-risk families in order to prevent the development of childhood weight problems and the intergenerational transmission of ill health. I would like to conduct further studies to find out what happens to these kids once they reach adolescence, and I hope that my research will help in the development of strategies to combat this serious public health issue."