Washington (CNN) The Trump administration will no longer seek to automatically release pregnant immigrants from detention -- a move in line with the overall efforts by the administration to hold far more immigrants in custody than its predecessors.

The change in policy could pave the way for more pregnant women to be held in detention facilities while they await lengthy court proceedings about whether they can stay in the US, facilities that are already decried by critics for tough conditions. The decision comes as immigration advocates have assailed the administration's efforts to hold more immigrants in detention writ large and its increased arrest of noncriminal immigrants.

The change in policy was sent by Immigrations and Customs Enforcement to Congress on Thursday morning and later announced in a conference call with reporters. The policy was first finalized in December, officials said.

According to Philip Miller, a top official in ICE's Enforcement and Removal Operations, as of March 20, there were 35 pregnant women in detention in ICE's custody. Since December, 506 pregnant women have been detained, Miller added.

He said it was difficult to estimate how many more pregnant immigrants would be detained under the new policy. Immigrants who are caught trying to cross the border illegally already are required to be in detention -- although once they pass a threshold test for asylum claims, the government can choose to release them. He also could not say how the policy applied to nursing mothers.

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