In my communication with clients, I always indicate that newborns photograph best between 5 to 10 days old. My go-to is to schedule the session 7 days after delivery. If the baby was born on the weekend, I'll be looking at the Friday or Monday closest to 7 days after birth. My reasons for this time frame are explained below:
Sleepiness is essential to a great session. While the occasional wide-awake baby will cooperate for their session, they will be swaddled the entire session and will not be posed in some of the more intricate poses. Exceptions are possible, and premature babies have a few special rules all of their own that I'll address a different blog post, but most newborns are quite sleepy during their first 10 days of life and will start to have more awake time soon after.
Newborn acne can escalate after 2 weeks. Some brand-new babies will have acne and some newborns will never have a single spot, but as a general guide, your baby's skin will have less baby acne before 10 days old. I do retouch images and eliminate as much acne as I can but it's always a better result when we avoid the typical baby breakouts, when possible. Also, babies who suffer with colic may not demonstrate tummy trouble until after 2 weeks, giving us an opportunity for a calm contented baby to photograph.
Newborns often have a growth spurt at 3 weeks which can make them very hungry, very frequently and will make a session challenging. A 3 week old baby in a growth spurt doesn't want to pose for photos. They just want to eat all day!
Every baby is so different but these generalizations have held true for the majority of newborns I've worked with in my business. I'm almost at 200 newborns!