I don't know. Ours was 9 pounds coming out, 13 at two months. She's going for her four-month checkup tomorrow and I expect she's 16-17 pounds. She'll probably break 20 before 8 months, and then it's time for a new car seat. She's a big healthy happy (mostly!) girl.
}:-)4
I expect Vlad to hit 20 lbs. before 6 months, but we'll keep him in the original car seat until his head hangs over the top, or he can sit up independently. The rear-facing vs. forward issue is really about how well they can sit up, which varies from one child to another.