We flew PDX-DTW on NW when our daughter was 6 weeks old in F in our lap (just about 1 year ago). No one asked for a birth certificate. I agree with others that, at that age, having access to materials under the seat in front is an advantage.

Usually babies do great flying... the vibration often lulls them off to sleep, just like being in the car. My daughter slept for 4 hours straight.

If you time the feeds right, you can do it during takeoff and descent, but even when we didn't do this, our daughter did just fine.

Probably the biggest risk traveling with one so young is that s/he may catch a virus from someone... but this is a risk anytime you're out in public with them. Try to limit exposure to those with runny noses and coughs!

I'm sure your trip will go just great. Good luck!