I have an Ergo, but have not used it for an infant yet. I don't necessarily think you need their insert, but you will need something to support the baby with until he/she is bigger. It does not support their heads at all and probably would be too big for a baby of 7 weeks. I just went ahead and ordered the insert so I don't have to worry about it, but I'm not due until January, so I haven't tested it out.
For other babies/parents, they find that the insert is useful to provide head support and body cushioning (to keep baby from falling out the side) up to the 4 to 5 month mark.
My recommendation would be to get it, try it both with insert and without, and return the insert if you don't need it.
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i got the insert and i think just folding a towel would have worked just as well. the baby shown in ergo's video explaining the thing is *tiny* and ds2 was never that small.
Here are some pics I found :
http://i49.photobucket.com/albums/f2...n/P1050057.jpg (close up of infant insert in action in infant side carry)
I hope you find something that works for you
mama to and, due in November