BOSTON — Parents will have to place their children in booster seats when in a car through the age of 7 or until they are 4 feet 9 inches tall, whichever comes first, under a new state law.

The law, which goes into effect in 90 days, was pushed by Sen. Steven Baddour, D-Methuen, and Rep. Michael Costello, D-Newburyport. It was signed into law late yesterday by Gov. Deval Patrick.

It replaces the current requirement that only children up to age 5 and weighing less than 40 pounds sit in a booster seat. It also requires children under 12 who are not required to use a booster seat to wear a seat belt.

"The purpose of this legislation is to save children's lives," Baddour said. "All good parents want to do what's right for their children."

"This is a very effective and very inexpensive way that parents can protect their children from potentially fatal accidents," Costello said. "It makes sense from a public safety perspective, and it makes sense from a public health perspective."

Booster seats allow children who are too large for a child car seat but too small to sit in a car seat to sit high enough to be strapped in safely with a seat belt. Seat belts on small children can cross the abdomen and neck, rather than over the hips and shoulders, causing injury in a crash.

Massachusetts is now the 39th state to require booster seats.

The federal government is providing states $25 million over the next four years to adopt booster seat laws. Massachusetts could get up to $750,000, Baddour said.

The bill was supported by AAA Southern New England, which had lobbied for the law since 2001, after the National Highway Transportation Safety Administration first endorsed them.

A 2002 booster seat bill was pushed by Baddour and passed the House but stalled in the Senate.