Car seat Recommendations.

The recommendations change every few years. 
See this American Academy of Pediatric Web site for
current recommendations.

AAP Car Seat Recommendations

"Now you have seen these standard recommendations, I would suggest you look at your child and use your judgment as to whether your baby should move up to the next seat. I had a patient of mine who weighed 40 lbs at 7 months old. There is no way he would fit in the seat for his weight. I think the seats should be for the length of the baby. If your baby's head and feet hit the ends of your present seat, then he probably needs a bigger seat.  The main idea is that the straps hit his body where they are supposed to make contact and hold him in the seat.   It's just common sense." .... Dr. Knapp