I split the difference between the newborn and small sizes on the Picky Pants pattern for newborns expected to be 8 lbs or more. My kids have been 9+ lbs and that size only fits briefly, but small is initially too big. My kids were born in March and I figured it would only be cold enough for longies for a few weeks anyway, so I'd go with what would fit right away instead of sizing up. Newborn size wouldn't have worked at all with my kids.