Evidence is growing that people's weight at three periods of childhood may be critical in setting them up to be fat in adulthood, scientists said.

Being born either too big or too small and the early appearance of puppy fat and tubbiness in teenagehood seem to be emerging as important factors influencing how much of a struggle with weight people have as adults, according to experts gathered this week at Europe's annual conference on obesity research.

Identifying these critical periods, they say, could help public health experts determine where best to intervene.

Many scientists believe that what happens around the time of birth is a particularly important time and that the evidence for this is especially robust.

Studies have shown that babies who are born large are more likely to end up fat as adults. However, being born very small also seems to increase the risk of obesity in adulthood, especially if such infants are then fed intensively to allow rapid growth so that they catch up with their peers.

"There are data from several different countries, including Israel, America, Europe and Southeast Asia showing a U-shaped curve," said Tim Lobstein, a childhood obesity specialist at the International Obesity Task Force.

"The lower end of it seems to be a thrifty situation where if you have a malnourished mother, the fetus will trigger the genes that conserve as much as possible. It will ... be triggering a laying down of any surplus energy as fat rapidly," Lobstein said.

"Babies that weigh less than 2.5 kilograms, or 5.5 lbs, would be considered small," he said. "The optimum is to try and have a baby around the 6 lbs or 7 lbs mark."

While the small baby problem is mostly one of the developing world, the major issue in wealthier countries is babies being born too big, experts say.

But convincing many parents, and even nurses, to move away from aiming for a big baby is likely to be a challenge, experts predict.

"They like to see them get high up on those (growth) curves, particularly in those early days. It's pretty ingrained in the maternal and child health nursing system to have a big baby, and it's probably not a smart idea," said Dr. Boyd Swinburn, an obesity expert from Deakin University in Melbourne, Australia. "And this underrecognition by parents is huge. It's going to be a major stumbling block."

The next stage that may be important is the preschool period, research suggests. Several studies indicate that children who gain weight before gaining height between toddlerhood and school-age seem to have a higher chance of being fat adults.

Rapid weight gain due to overfeeding in the first year of life may be particularly risky for later obesity, experts say.

Major studies over the last few years indicate that about one in three children who are fat in early childhood end up as fat adults. Children that get fat before the age of 8 tend to end more severely obese as adults than those who gain their weight afterward.

But being fat in the teenage years seems to be even more predictive of later risk of obesity, research indicates. About 70 percent of fat adolescents end up with obesity later in life.

The problem with obesity in adolescence seems to be that the male sex hormone testosterone pushes fat to the belly — a high risk location, while in girls, the problem is that they tend to gain alot of weight during their teens.

"We know that fatness in adolescence predicts later obesity, but the link between adult obesity and becoming fat in adolescence is less clear," said Dr. William H. Dietz, director of nutrition and physical activity at the U.S. Centers for Disease Control and Prevention.

The three stages of childhood considered critical for obesity development outlined at the conference are scheduled to be discussed at an upcoming World Health Organization expert meeting in Japan later this month.