World's fattest baby: Mum reveals 10-month-old baby is size of a 9-year-old

A Mexican mother whose 10-month-old son weighs the same as a nine-year-old has spoken out about his mystery condition.

Isabel Pantoja, 24, from the Pacific coast state of Colima, said at first she assumed the baby Luis's growth was down to the fact that "I had good breast milk" but now she is desperately worried for her son.

She also revealed the family take it in turns to push the boy to a local hospital where doctors are baffled by his condition.

One of the main hypotheses is that the boy could have a disease called Prader-Willi Syndrome, a genetic condition in which children have an insatiable appetite and weak muscle tone.

Ten-month-old Luis Gonzales and his mother Isabel Pantoja, 24, are pictured at their home in Mexico. Photo / AFP

However, Luis does not eat voraciously or spend the day demanding food.

Medics are considering giving him with hormone treatment that can cost $US555 per shot, despite his father earning just $200 per month.

At birth Luis weighed 3.5kg (8lbs), about the same as his brother Mario, who is almost three but is now dwarfed by his baby brother.

At two months of age, Luis weighed 10kg (22lbs). Over the next eight months he gained a staggering 18kg (40lbs).

His parents have since created a Facebook page and opened a bank account for people to donate money for Luis Manuel's medical care.

They take turns pushing him to the hospital in a decrepit stroller for daily blood tests.

"It hurts to watch the nurses search among the rolls of fat on his arms for a vein," said father Mario Gonzales.

Pantoja caresses her son as she recounts that when he was a little more than a month old, the boy had to wear clothes meant for two- or three-year-olds.

Now he has to go to the hospital as many as four times a week for tests. One day the stroller simply collapsed under his weight.

Doctors are still not certain of the cause of Luis weight gain. Photo / AFP

The excess weight means he cannot walk or even crawl and can only sit upright.

Pantoja said he is becoming exhausted from carrying him so much, despite being a well-built man who works at a juice plant.

But there is hope. Silvia Orozco, a surgeon who specializes in nutrition, contacted the family and examined the boy.

She concluded that his life might be in danger, but is awaiting the results of tissue sample analyses in the United States.

Orozco said that, rather than Prader-Willi Syndrome, his problem may be this: while pregnant with Luis Manuel, his mother's diet lacked certain nutrients and this caused glands that manage his metabolism to underperform.

If that turns out to be the case, the prognosis is good and would involve hormone shots.