My son weighs 14lbs 11 oz and he fits a little big in the size 3-6 months. This does work like magic. You do have to train your baby to sleep through the night but this helps calm the reflex and it prevents him from waking at night. I recommend it to friends but also should read the 12 hours of sleep by 12 weeks old book. My baby sleeps 630pm to 630am with this magic suit.