A recreational drug taken during pregnancy can lead to underweight babies, according to a study.
Methamphetamine is also known as crystal meth
A US team studied 1,618 newborns and found they were 3.5 times more likely to be underweight if their mother had used methamphetamine during pregnancy.
Methamphetamine, also known as crystal meth, is a powerful stimulant and is growing in popularity in the US.
The researchers, writing in the journal Pediatrics, said the babies could have further "developmental implications".
Previous studies have shown low birthweights are linked to behavioural problems and learning difficulties in childhood, and heart disease and type 2 diabetes in later life.
The researchers looked at 1,618 newborns from four different medical centres, located in Oklahoma, Iowa, Los Angeles and Honolulu.
They asked the babies' mothers if they had used methamphetamine during pregnancy, and also analysed the babies' stools for presence of the drug.
They found 84 of the newborns had been exposed to the drug while in the womb.
The researchers then adjusted the data to take into account other factors that could cause low birthweight, such as socioeconomic status, smoking and alcohol intake.
They discovered that newborns whose mothers had taken methamphetamine were 3.5 times more likely to weigh less than 5lb (2kg) than the babies who had not been exposed to the drug.
The scientists said they believed methamphetamine was restricting the flow of nutrients from the mother to the child in the womb, thereby restricting the developing foetus's growth.
Professor Barry Lester, director of the Brown University Center for the Study of Children at Risk and an author on the paper, said: "We are very concerned about the developmental implications.
"Children who are born underweight tend to have behaviour problems, such as hyperactivity or short attention span, as well as learning difficulties."
But Professor Lester cautioned: "While these children may have some serious health and developmental challenges, there is no automatic need to label them as damaged and remove them from their biological mothers."
The children will now be followed by the team until they are three years old.
Previous research from the same team found that crack cocaine had a similar effect on a baby's weight.
Harry Shapiro, a spokesman for Drugscope, said: "Methamphetamine, also called crystal meth and crank, is a very powerful stimulant drug.
"It is very much an American phenomenon; it isn't so popular in other countries such as the UK."
He said the study was interesting but cautioned that it was very difficult to fully adjust data to take into account all other factors that may cause restricted growth.
But he added: "There is no doubt if you are using a powerful stimulant while you are pregnant there will risks to the baby."