The parents of an obese 18-month-old toddler can't explain her uncontrollable weight gain and are pleading with doctors for help.

Cheng Lichao and his wife Feng Xiaochun took their daughter Dan Dan to a local hospital in China weighing in at an incredible 19kg.

At 18 months, Dan Dan is double the standard weight for her height and age, and is still unable to walk, possibly because of her size.

Dan Dan's parents claim they have no idea why their daughter has grown so large, and have been to a number of hospitals to seek help.

"She started to grow quickly from one month old. When she was one-year-old her weight had reached 15kg," Dan Dan's mother told a local news agency.

"She has a big appetite. Each meal she must eat nearly a big bowl of rice and drink a lot of water."

The couple also has a five-year-old daughter who weighs six kilograms less than her younger sister.

They say there are no other members in their family are obese, and are searching for a medical explanation for their daughter's weight.