The Beco Gemini can be used from birth on up.
It's got a head-support flap similar to the Baby Bjorn (profile is similar to the Bjorn, but it's more versatile once baby gets larger - it's got back & hip carry options!) and the ability to cinch up the carrier body to allow you to carry a smaller baby on your front (facing inwards, legs froggy style) hands-free.
Another option for the itty-bitty phase is a wrap carrier like a Moby (stretchy) or GypsyMama/Wrapsody (woven), but there's a learning curve to getting it right. I love my wrap (EllaRoo, which is another woven wrap brand) but it makes my DH uncomfortable seeing me wear the baby in it.
You could also stay with the Beco brand and get the Butterfly, which has a built-in infant insert & does front and back carries as well. I didn't choose the Butterfly myself (read about it and thought it sounded better for more petite mamas ... I can use a lot of terms to describe myself, but petite is not one of them!) and went with the EllaRoo and an ErgoBaby for different phases of development.
Hope this helps a little!
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Liz
DD (3/2010)
"Make mistakes! Get messy!" - Miss Frizzle