Under the new law, all children under 2 years old must face the rear in their car seat.
Children who do face forward must be harnessed until they are 55 pounds instead of 40 pounds.
The law also clarifies that children must be in a booster seat until they are 8 years old, are 57 inches tall or weigh 80 pounds.
A spokeswoman for Safe Kids Maine said the rear-facing requirement is one of the most important changes to the law.
"We have a lot of issues with children who are turned forward too soon and we've seen the results of those crashes from children suffering internal decapitation, head and spine injuries, that could have been prevented if they're facing rear and the seat itself takes the brunt of the crash for use instead of the child," said Staci Fortunato.
The law takes effect 90 days after the legislature adjorns.