SAN FRANCISCO (Reuters) - U.S. President Donald Trump’s administration said on Thursday it would no longer presume that many pregnant women detained by immigration authorities should be released from custody, reversing an Obama-era directive.

A Homeland Security Immigration and Customs Enforcement (ICE) bus is seen parked outside a federal jail in San Diego, California, U.S. October 19, 2017. REUTERS/Mike Blake

U.S. Immigration and Customs Enforcement officers will make a case by case determination under the new policy. Women in their third trimester will still be released as before, said Philip Miller, an ICE deputy executive associate director.

“Just like there are men who commit heinous acts violent acts, so too have we had women in custody that commit heinous acts,” Miller told reporters on Thursday.

The Republican president has vowed to crack down on illegal immigration, including policies on which deportees can remain free during pending cases. Democrats and advocacy groups have criticized the administration for separating migrant from their children when detained.

During President Barack Obama’s administration, ICE in 2016 announced that pregnant women not subject to mandatory detention should be presumptively released.

Miller said on Thursday the new directive was meant to align with a Trump executive order mandating tougher ICE enforcement. Thirty-five pregnant women are in ICE custody, all subject to mandatory detention, he said.

Since the policy was implemented in December, Miller said, 506 pregnant women have been detained by ICE. He could not say what happened to each of them, but noted that some likely had been deported while others might have been released in the United States.

Michelle Brané, director of the Migrant Rights and Justice Program with the Women’s Refugee Commission, criticized the move and said many women entering the United States are pregnant due to rape.

“Detention is especially traumatic for pregnant women and even more so for victims of rape and gender-based violence,” she said in a statement.

Miller said pregnant women with asylum claims determined to be based on a “credible fear” of persecution in their home country would still likely be released.

The policy change was first reported by The Hill on Thursday, citing internal ICE documents.