As soon as I could get the bike out of the box and built up.

Seriously, it depends on what your cycling fitness is like now and what it's like that month before you deliver. Also, whether or not you can even tolerate the pressure on your peritineum after the delivery. If you chose to do it, I would definitely pick the 16 miler.

Other practical things to consider... Who will care for the baby while you ride? What about breast feeding?

I am not a Mom, just a healthcare person. So I'll defer to all the great Mom's out there to point out the millions of things that I would never even think of that might be an issue.