A man has been arrested after crashing a car ‘while having sex with a woman whose baby was strapped in the backseat’.

The couple sustained multiple injuries after the vehicle slammed into a tree on a highway in Washington State in the US, reports the Mirror.

Michael Tonkin, 23, was allegedly drunk behind the wheel when Daisy LaRoque sat on top of him to engage in sexual activity, State Trooper Brooke Bova said.

Shocked witnesses said they saw the pair emerge fully naked from the car following the collision.

LaRoque, 23, broke her pelvis while Tonkin broke his wrist.

The three-month-old child, properly restrained in a car seat in the back, was uninjured.

Witnesses told police they saw the couple exit the vehicle naked and attempt to flee the scene.

Bova believes the couple were scrambling to locate their clothes before police arrived.

If Tonkin is convicted, it will be his fourth DUI in Washington State - a felony offence.

He has also been charged with vehicular assault and child engagement.

He was booked into Pierce County jail immediately after his release from the emergency room.

Laroque has been admitted to the hospital and it is unclear whether charges will be brought against her.