Amber Brewington blamed "severe" post-partum depression for her decision to inject her infant son with salt water while he was in the care of Children's Hospital of Pittsburgh and, before that, a hospital in Nashville, Tenn., according to Pittsburgh police.

But the 21-year-old Tennessee woman, who has been charged with attempted homicide, also was enduring a tumultuous relationship with the boy's father, Terry King.

Court records from Hickman County, Tenn., where the couple live, show that Ms. Brewington has accused Mr. King, 39, of brutally assaulting her on at least two occasions, both while she was pregnant.

In October 2006, Ms. Brewington filed for a protection order against Mr. King, claiming he choked her until she lost consciousness. Ms. Brewington was pregnant with their first child at the time. Several weeks later, she asked for the protection order to be removed.

Almost a year later, Mr. King hit Ms. Brewington in the head during an argument, according to an affidavit. When a female neighbor tried to intervene, Mr. King punched her in the face.

Ms. Brewington was again pregnant, this time with Noah King, who is now 4 months old and is in critical condition at Children's Hospital.

Ms. Brewington was arrested Tuesday night when she admitted to police that she injected salt water into her son's feeding tube, according to a criminal complaint. She also faces charges of aggravated assault and endangering the welfare of a child.

A judge has ordered her to undergo a psychiatric evaluation at the Allegheny County Jail. Her two other children, including one from a prior relationship, are being cared for by a relative in Tennessee.

On Wednesday, a pediatrician at Children's Hospital talked to reporters about Munchausen syndrome by proxy, or MSP, a mental illness that causes parents to fake or even cause a child's medical problems, often to get attention.

Neither hospital officials nor police could say if Ms. Brewington suffers from MSP, which is difficult to diagnose.

Dr. Johanna Goldfarb, a pediatrician at the Cleveland Clinic, said women who have the condition often struggle with bad marriages and were themselves abused as children.

Ms. Brewington has no criminal background in Hickman County, but Mr. King has faced charges there on almost a dozen occasions.

In 2001, he was arrested after he attacked his mother at her home and a neighbor's house, an affidavit said. He later pleaded guilty to disorderly conduct.

The following year, he pleaded guilty to domestic violence after a fight with another girlfriend. The woman told police he had been drinking and had ordered her to have sex with him. When she refused and tried to leave, he grabbed her, pushed her against a kitchen cabinet and started choking her with both hands. The woman's 6-year-old son called police.

In 2005, the woman told police Mr. King threatened her with a knife, telling her she was "going to die."

By 2006, Mr. King was with Ms. Brewington.

Mr. King accompanied Noah to Pittsburgh last week, when the boy was transferred from the Vanderbilt University Medical Center in Nashville to Children's Hospital in Oakland.

The boy had an unusually high level of sodium in his system when he arrived here. He seemed to be improving until Ms. Brewington visited him on Sunday. When she returned two days later, a nurse saw her tampering with the baby's feeding tube.

Pittsburgh police also have questioned Mr. King, but he hasn't been charged with any crimes.