Moms-to-be that gain more than 40 pounds (18kg) during pregnancy are at greater risk of health problems during birth, it has emerged.

Babies over the average of nine pounds risk mother’s and baby’s health, Dr Teresa Hillier of Kaiser Permanente Center for Health Research in Oregan told Reuters.

"Because there are so many women who are gaining more than 40 pounds during pregnancy, it’s an important health message for most women to avoid excessive weight during pregnancy," the doctor said.

Hillier’s team looked at medical records of 41,540 women who gave birth in Washington, Oregon and Hawaii from 1995-2003.

These babies may be programmed to become overweight or obese in later life studies have suggested.

According to the National Academy of Sciences, expectant mothers should gain between 13 and 39 pounds during pregnancy, Web4Health reported.