Montreal children are fatter than those living outside the metropolitan area, according to a new study from Quebec's statistics institute.

The study tracked 2,120 children from birth to age seven, and found more than one-fifth of Montreal children, or 21.3 per cent, are overweight.

Children who were deemed overweight at age seven most often showed signs of obesity by the age of four.

The Montreal rate is higher compared with other Quebec regions, where only 13.4 per cent of children of the same age are overweight or obese.

Poverty is a major factor in obesity, and a greater proportion of children who live on the island don't have access to the same food or living conditions as those in other Quebec cities, said Lise Dubois, the study's co-author.

"Families who are poor would need a car to go to the large grocery centres to get very good prices. But often they don't have a car and can't go there, so there's segregation" in terms of access to healthy food, said Dubois, who is an epidemiology professor at the University of Ottawa.

Child obesity is a worrisome trend because of associated health risks such as Type 2 diabetes, which is being diagnosed with increased frequency among younger children, Dubois said.

"These children may have a lower life expectancy than their parents, because they beginning to accumulate risk factors for their health very early in life," she said.

Other factors, such as smoking during pregnancy, can affect a child's weight, Dubois said. When a mother smokes during her pregnancy, her child has a 21.6 per cent chance of being overweight by the age of seven, compared with 13.4 per cent for children born to non-smoking mothers.

Exposure to cigarette smoke affects a fetus's metabolism, according to some research.

Children who don't sleep enough, consume soft drinks or don't eat enough vegetables are also at risk of becoming fat, the Quebec study concluded.

Obese parents also carry a greater risk of having overweight offspring.

The study's conclusions will be used for public education about the risks of childhood obesity, Dubois said.