Sizing by *length* is recommended. Intended for babies who enjoy fetal position. Sizing fits most.

SIZE WEIGHT LENGTH
xsmall preemie to 7 lbs up to 21"
small 7 to 12 lbs 21-23"
medium 12-18 lbs 23-25"
large 18-22 lbs 25-27"

Length is taken from head-to-toe. Size guidelines are an approximate fit range.