we converted ours a bit later as DD was on the small side. i think we converted closer to 7 months. just as a guide at her 12 month weigh in she was 7.9kgs (2.7kgs at birth). just to give an idea...so i *think* she may have only been around the 5ish mark at 6 months....

because her body was so tiny her legs didnt sit right in the seat and also she was too tiny to be properly strapped in by the harness. but by 7 months it worked great.

HTH