What is your most effective lower abdominal exercise for the dreaded "baby bulge"?

One you will do. The most effective exercise is the one you'll do regularly. Check out these exercises for strengthening your abdominal muscles. If you're not used to doing these kinds of exercises, start with the ones for beginners: http://www.Acefitness.Org/exerciselibrary/exercises.Aspx?Bodypart=1.