MATAMOROS, Mexico — Griselda was 38 weeks pregnant when she sneaked across the Rio Grande into the United States late one night last year. She started having contractions in a Border Patrol facility in McAllen, Texas, and was taken to a hospital where the medical staff gave her an injection to calm her pain and stop her from going into early labor.

Two days later, she was on a crowded bus back to Mexico, moving into a tent camp with hundreds of other migrants who were waiting for permission to enter the United States. When she finally gave birth 10 days later, her tiny daughter joined her there until a local shelter made room for them.

The wide-ranging new controls on immigration put into place under the Trump administration have made it harder for migrants of all kinds to cross the Southern border, but they have been particularly difficult for pregnant women, who often arrive at the border after arduous journeys, in a state of exhaustion.

Previously, many such women would be allowed to petition for asylum and give birth in safety in the United States while their cases were being considered. But most now, like Griselda, are sent swiftly back to Mexico to take their chances in crowded shelters and filthy tent camps. Some are held in U.S. detention facilities for months.