A baby is fighting for life in hospital, its mother is dead and a man has been arrested after an incident in Melbourne's north-west on Saturday afternoon.

Key points: The woman was pregnant when she fell from a moving car

The woman was pregnant when she fell from a moving car Police said a 35-year-old man was assisting homicide investigators with their inquiries

Police said a 35-year-old man was assisting homicide investigators with their inquiries The baby remained in hospital on Saturday evening

Police said it was believed a man was threatening to assault the woman at a Kiwi Retreat home in Keilor Downs just before 1:00pm.

"The incident continued in a car and the victim has fallen onto the road as the car was driving around a corner," a police spokeswoman said.

The pregnant 32-year-old was airlifted to the Royal Melbourne Hospital after 1:00pm in a critical condition.

Police said she died in hospital on Saturday evening.

The baby, delivered via caesarean section, remained in hospital in a critical condition.

A 35-year-old Keilor Downs man was assisting police with their enquiries on Saturday night.

Police earlier said it was believed the two were known to each other.

Homicide detectives were investigating.

Belongings were strewn across the road near the scene of the incident on Saturday afternoon.

The ABC understands the woman had two other young children under the age of five.

Victorian Minister for Transport Infrastructure Jacinta Allan said the incident was a "tragedy".

Flowers have been laid in Keilor Downs where the incident happened yesterday. ( ABC News: Emilia Terzon )

"My thoughts are with the family and the loved ones of the woman and her child," she said.

"This is such an awful tragedy.

"Also too a thought should go out to the paramedics, the nurses, the doctors who treated the mother and her child in these awful circumstances."

Gurdev Rayat said she had never seen another incident like it. ( ABC News: Emilia Terzon )

Neighbour Gurdev Rayat said she was shocked to see such an incident in the neighbourhood.

"Over 31 years I have been in Melbourne, I have never seen anything like this, especially in this area," she said.

"I never heard anything like this happening and especially to have it happen across the road … it's shocking and it makes you think twice now."

Neighbour Ivan Sucic said he was watering his flowers when he heard a woman screaming.

He said he turned around and saw something lying on street. When he walked over to investigate he saw it was the woman.

"When the ambulance came they have done the CPR on the spot," he said.

"[I have] feelings I can't explain, I feel sorrow."

Anyone with information should contact Crime Stoppers on 1800 333 000 or submit a confidential report online.