A New Zealand medical student who used a Taser to repeatedly shock a nine-month pregnant woman, causing her to go into labour, has been charged with attempted first-degree murder in the United States.

Christine Renee DeVaux, 38, on leave from medical studies in New Zealand, is alleged to have sneaked into the vacant Maryland home of her boyfriend's pregnant daughter-in-law at the weekend.

She wore a surgical cap and cloth gloves, laid plastic liners across the kitchen floor, and lay in wait for her victim, Lynae Mattson, 29.

When Mrs Mattson arrived home, DeVaux allegedly shot her with a Taser and beat her for 30 minutes, causing her to go into labour, US authorities told the Washington Post.

Lynae Mattson, 29, and her almost full-term baby survived, and were recovering at home near Damascus, Maryland, about 65km northwest of Washington DC, police said.

"We're lucky we're not here on a homicide case," Montgomery county deputy state attorney John Maloney said in court yesterday.

According to court documents released yesterday (NZ time), Mrs Mattson escaped by pulling her attacker's hair and running to a neighbour for help.

She spent about 12 hours in a hospital being treated for injuries, including scratches and bruises.

Doctors were able to prevent a premature birth.

US newspapers reported yesterday that DeVaux was driven by jealousy and rage, and thought Mrs Mattson's husband and the pregnancy were the source of problems between her and her own boyfriend, Bruce Mattson - the victim's father-in-law.

The court documents said Mrs Mattson, asked during the attack if DeVaux "really wanted to murder her and her unborn child".

DeVaux allegedly replied: "I never wanted it to come to this."

DeVaux told Mrs Mattson that her growing family was keeping DeVaux away from Bruce Mattson. Devaux refused Mrs Mattson's pleas to let her go.

"You're going to press charges," DeVaux said.

DeVaux appeared to have worked in Wellington for Standards New Zealand in 2004, after coming here from the United States.

She was reportedly on leave from her medical studies in New Zealand, after being injured in a bicycle crash.

DeVaux was charged with attempted murder and other counts, and was not granted bail because she was deemed a flight risk, given her strong contacts with New Zealand.

The Taser was just one weapon police recovered, documents revealed.

Investigators also found about 15m of wiring and cord and pepper spray in the victim's home.

Police also found a .22 rifle, a Crossman BB air rifle and 150 rounds of ammunition at Bruce Mattson's residence, where they said DeVaux fled after the attack.

DeVaux was taken into custody on Friday night at a Frederick, Maryland, motel, where she had rented a room.

DeVaux worked at the US National Institute of Standards and Technology in Gaithersburg, near Washington, from 1997 to 2004 as a standards specialist, and has written papers touching on technical infrastructure in New Zealand.

Her preliminary hearing is set for for July 2 in Montgomery County District Court.

The victim's husband, Ben Mattson, an FBI agent, declined to comment. Police said there was no indication that the attack was related to his job.

- STAFF REPORTER AND AGENCIES