US state laws used to prosecute pregnant women who take drugs are stopping them seeking help from health services and are a violation of their rights, Amnesty International said on Tuesday.

The human rights group sounded the alarm in a report about the increasing practice of charging pregnant women addicted to drugs with endangering their own fetus.

It said this had undermined efforts in recent years to treat the addiction of possibly thousands of women. There is no figure on the number of women charged with a crime related to their pregnancy, the report said, but measures to crack down on those who take drugs have been proliferating.

"Drug dependence is a health issue but now authorities are punishing people for their condition, treating it solely as a crime," the report's author Carrie Eisert said in a phone interview.

"These harsh and discriminatory laws are making pregnancy more dangerous and trampling on human rights in the process."

Amnesty said 38 of 50 US states – from Mississippi to South Carolina – have laws considering unborn babies as potential victims of a crime. About half that number consider substance abuse during pregnancy to be child abuse.

Such lawmaking has been gaining pace, Amnesty said, with legislators in 17 states this year alone having introduced similar measures. These include New Hampshire, Rhode Island and Indiana, according to an analysis by Amnesty of data by the New York–based Guttmacher Institute, a reproductive rights organisation.

Amnesty called on states to repeal laws that make women liable for endangering their own pregnancies, which it said drove them away from prenatal care and even drug treatment services out of fear of punishment.

Prosecutions of pregnant women who were addicted to drugs have in the past resulted in mothers losing custody of their child after birth, the group's research showed.

Other women have been incarcerated, Eisert said.

"Criminal justice has no place in public health," she told the Thomson Reuters Foundation.

In 2014, Tennessee was the first state to introduce a law that explicitly sent new mothers to jail for substance abuse, Amnesty said. The measure which expired in July 2016 was proposed again by lawmakers earlier this year, though unsuccessfully, it said.

A drug–addicted baby is born every 25 minutes in the United States, with the number of infants experiencing opioid withdrawal increasing nearly 5-fold in the past decade, according to a study published this year.

—Sebastien Malo