Everything below is written for breastfeeding.
The fourth week often turns a corner — feeds go down a little easier and the first real smile is getting close.
No measuring. Feed whenever they ask, both sides offered, and let them finish. If it hurts every time, ask for latch help now.
Draft figures for this age, pending clinical review.
Trust the worry. None of this is an overreaction.
If you want the detail
The first real social smile — usually by 6–8 weeks. Better tracking of faces and objects with the eyes.
Drafted from public sources — see the full source list. This is not medical advice — call your pediatrician with any concern.