This mum-of-three has shared a simple car seat trick that could save your child's life.

A mum’s message on car seat safety has been shared 48,000 times after she posted it on Facebook.

Mum of three Rachel McNamara, from Maine in the United States, wrote that parents should be sure that every time they strap their child in for a car trip, they are safe.

“After strapping your child into their car seat, ask yourself if you’d be comfortable flipping it upside down,” she wrote.

"Remember that the chest clip should be at armpit level and the straps should be tight enough to pass the pinch test."

Rachel added that when a child is buckled in and the chest clip is in the proper place, "you shouldn't be able to pinch any of the strap between two fingers".

"If you can pinch the strap here's what to do: move the chest clip back down, tighten the straps, move the chest clip back up and test again."

After her post went viral, Rachel clarified that she wasn't suggesting parents should flip their kids upside down.

"Just that they should feel confident that their child would be safe IF the car seat was flipped over in an accident," she wrote.

She also suggested that despite her tips, parents should read and follow the instructions that come with every car seat, as well as the car manual, on how to install the seat properly.

Many parents commented that Rachel's post would help save the lives of hundreds of children around the world.

"I seriously can't love this enough," one mother wrote.

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"As a grandmother I needed this reminder! Thank you! Keeping our kids safe is what it's all about!" said another.

Other shared stories of how a properly-installed child seat had saved their children's lives in an accident.

"My mum and son just went through a head on collision over a week ago where the driver in the other car fell asleep and hit them," one woman shared.

"My son was properly restrained in his car seat. If he wasn't I don't even want to imagine how the outcome could have been. So thank you for this post!"