Infant carriers

I have a carrier/backpack thing for my baby that would really make it easier to get things done since my little one wants to be held constantly. However it says the baby needs to be 8 lbs before she can use it and after 4 weeks she is still only 6 1/2 lbs. Has anyone else used one of these for an infant so small? I know the slings can be used with any weight, but this carrier specifically said 8 lbs and up.