You could work this in any thickness of yarn, though the heavier weights are probably not suitable for the smaller babies. For 4 ply and an E or F (3.5mm or 4mm) hook, I would get a 17” wide blanket from 90 chain to start. The best thing is to play with the pattern and see what suits you best.
Chain an Even Number
Foundation Row: 1 tr in 4th chain from hook and in each chain to the end. Chain 1, Turn.
Row 1: 1 dbl. in first tr., chain 2, 1 dbl. in space between 2nd and 3rd tr. *chain 2, 1 dbl. in space after next 2 tr.*
Repeat from * ending chain 2, 1 dbl. in top of turning chain, Chain 2, Turn.
Row 2: 1 tr. under first chain 2, *2 tr. under next chain 2*
Repeat from * ending 1 tr. under last chain 2, 1 tr. in last tr. Chain 1, Turn
Repeat rows 1 and 2 until desired length.
Last Row, treble evenly across, Fasten off.