(CBSNews) -- The safest place for a child in the car is in a car seat.

But many parents don't know that car seats expire.

Millions of outdated car seats may be in vehicles across the country right now.

"I never really thought anything of it, we used it for about a year for each kid, it never had been in a car accident it was in good shape so I just reused it," Mother Samantha Booth said. 

Just one problem -- she didnt know it was already a year past its expiration date when she used it for her son nathaniel.

"I was shocked and relieved and I felt guilty that I had my youngest in an expired car seat," she said.

According to safe kids worldwide, 3 out of 4 car seats are used incorrectly. That includes the use of expired car seats.

"In general we are looking at 6 year expiration date and that's from a manufacture date not the date you purchased it or the date you started using it," American Academy of Pediatrics car seat expert Kristen Urso said.

You can find the expiration date on the bottom of your car seat. Look for the sticker that has the manufactured date and the do not use after date. Manufacturers put expiration dates on car seats because the plastic they're made of can break down over time.