A pregnant woman has been attacked with a hammer by four masked men in a terrifying armed robbery in Melbourne's south-east.

Her attackers returned her mobile phone only after she told them it contained ultrasound photos of her unborn child.

The 24-year-old woman was walking along Walsh Street in Noble Park about 11.30pm on Tuesday, towards the intersection of Simpson Street, when the four men in a blue Ford Falcon XR 6 whistled at her.

Police said the woman quickly walked along Simpson Street and turned right onto Myrtle Street, but the men made a U-turn and drove up behind her.

The foursome pursued her down the street, before getting out of the car and threatening her.