Babies born in the US are lightweight and reasons for this are not certain.

A study stated that babies’ birth weight in full-term between 1990 and 2005 declined by about two ounces. This started reversing the trend of birth weight climbing since the 1950s.

A data that was analyzed by researchers showed that there was a 1 per cent increase in babies weighing less at the time of birth. It was also established that the birth-weight decline didn't stop in 2005.

Emily Oken, an author of the paper and an assistant professor of population medicine at Harvard Medical School, said that, “The results are surprising and unexpected. We do need to keep a close eye on the babies that are the smallest babies to see if the proportion of those is increasing over time, because they may require extra resources and have extra health concerns.”

These findings call for attention from medical professionals for babies born around 37 weeks and 38 weeks, and babies that are born pre-term.

The report published in the February issue of Obstetrics & Gynecology said mothers who had uncomplicated vaginal deliveries had babies weighing 2.78 ounces less. These women were white, well-educated, married, did not smoke, and received prenatal care.

Experts also said that babies who were heavy were likely to have obesity.