Babies
They don't take direction, they don't care that there is a camera in their face,and they can have good & bad days. 

For photography:
Newborns,  are considered 1-4 weeks old. This age is key because all babies wants to do is sleep, eat, and poo. At this age, they still like to be squished up and their skin hasn't reacted to the air yet. Over a month old we can still take adorable photos, but baby might not be the tiny little bundle anymore.

If you missed the newborn phase here are some other moments to consider... at 3 months they are smiling and reacting to everything. At 6 months they might be sitting up and a great milestone to capture.
Older than 6 months I suggest a 1 year session.
Let's chat about your little babe and see what kind of session is best.