The star wraps are made to a generous fit so 4-10 months wraps will be in most cases too large for babies under 3 months. If you are buying for a new baby the 0-4 is definitely a better bet. For a guide use the measurements below measuring your baby from neck to foot (measurement B) and neck to nappy (measurement A).
Small (Size 1) = 0-4 months
Large (Size 2) = 4-10 months
(Size 3) 10-18 months - COMING SOON!