Generally I like to photograph newborns around 2 weeks old. However, if the baby is pretty small I will wait until they've hit near 8lbs. In my experience, 8lb babies sleep better during a shoot than 6lb babies. This little lady didn't weigh 8lbs until near 4 weeks old. 4 weeks is the nerve-racking age because you never know if they're going to sleep or be awake if you pull and pose them. Not only that, but this lady was sooo tall. I this she beats the heights of any newborn I've ever photographed. I think mom and dad said she was born at 22-23 inches in length.
Needless to say, we didn't have ANY issues getting her to sleep during her session.