Switching to forward-facing ones before that age is dangerous and could result in spinal injury in an accident.
In the UK it is common practice to switch babies to a front-facing seat when they weight 9kg (20lb) – around the age of eight months for an average boy.
The study, in the British Medical Journal, was backed by safety experts.
Dr Elizabeth Watson, a GP, and DR Michael Monteiro, who specialises in facial injuries at the Royal Surrey County Hospital, Guilford, say several studies show this is the safest.
They cite evidence from Sweden, where using a rear-facing seat up to the age of four is common practice.
Dr Watson said: “Rear facing car seats cradle a child in an impact with any frontal component, and align the head, neck and spine, spreading the crash forces over all of these body areas.
“In a forward facing car seat, a child’s body is held back by the straps, while the head keeps moving forwards, and the relatively large head mass and differences in the cervical spine in young children can lead to excessive stretching of the spinal cord.”
A U.S study concluded that rear-facing seats were more effective in protecting children aged 0-23 months in a crash.
By Tasha Usefnia