DS was around 4.5 months when I stopped using it. By that time he had really good head control. He actually liked the insert, so I didn't have any reason to stop using it.
I don't think the size matters. DD hit 16 lbs at 2 months and we continued to use it until she had good head control. Otherwise, her head just flopped around because it stuck out way above the end of the fabric (if that makes sense).
We have never used the insert. He was over 10 lbs when we got it and since it was for babies up to 12 lbs I didn't see the point. His head did not stick out and wobble though, He hung out nice and low so his head was supported. The place I bought it from also suggested I swaddle him instead of using the insert - since I was worried about him getting hot in it.
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Re: ERGO users - when did you stop using infant insert?