Can you re use a car seat?

So I had my son in November 2015, and here I am due again with my 3rd baby November 2016..& I was wondering what the rules were on re using a car seat! It's an all black car seat so the color isn't a problem wether the baby be a girl or boy, & My son will be in a bigger car seat once it's time for me to have the new baby. I've heard that your not suppose to use the same car seat from a previous child. I just don't understand why and would like some opinions from other mommy's!