Short sleeve romper from Old Navy. Brand new with tags.

Size chart:

0-3 months fits up to 23 inches & 7-12 pounds
3-6 months fits 23-27 inches & 12-17 pounds
6-12 months fits 27-29 inches & 17-22 pounds
12-18 months fits 29-31 inches & 22-27 pounds
18-24 months fits 31-33 inches & 27-30 pounds