I purchase this halo sleep sack for my grandson , who be now 4 month old . I order small , and I be glad I do ; he be about 15 lb and 25-27 inch long . the small size be actually a wee bit big on he ; he have plenty of room to grow into it and I can see this fitting he at 9 month old ; there be ample room for he to kick he little leg . he wear this with a long-sleeved onesie underneath during cooler weather , and with a light blanket , it be just right for chilly fall evening without be too warm . the fabric be lovely and soft , lightweight but definitely not flimsy . I love the fact that this zip from the bottom ; it make for super-fast diaper change without expose he whole body to the cooler night air and cause discomfort . I also order this in the microfleece fabric ; however , I prefer this organic cotton sleepsack , because the microfleece be somewhat heavier and I can not see he wear the microfleece unless we experience sub-zero temperature outside and the house be drafty ; yes , it be that warm . all in all , I highly recommend this cotton sleepsack for those who live in climate with cooler fall\/winters .