I put it on the first set of slots. She almost could have used the second set. They were maybe a smidge over her shoulders, but I would have been comfortable using them. Funny, considering the lowest setting on her Shuttle 33 is still about an inch above her shoulders. I think a preemie would fit really well in this seat.
The only thing I might be a bit hesitant on is that the infant insert doesn't click in to anything. It just kinda nests under the headwings, then the straps go through it and there's a cut out for the crotch buckle. I suppose the weight of a child could counteract and keep it in place, but I think a more "secure" attachment would have been nice. And I'm assuming the insert can't be used FF, but I didn't get a chance to read the manual (my 16m DD was running around).