The flights will be from Philadelphia to London (I connect via British Airways to Pisa) and then returning to Philadelphia from Rome. Both of the long flights are on 333s. The flight to London is an overnight flight. I don't have any built up miles as I have not flown in a long time.
I understand there are no guarantees but I would like to optimize my chances. It's hard enough to get comfortable in the seats without having an additional person squirming around inside trying to get comfortable too.

Thanks for any advice you give me.

MSPeconomist - The basis for the upgrade is that pregnant women suffer from a wealth of health issues due to pregnancy. Not the least of which is a higher risk of blood clots in the legs when flying, severe back issues from the extra weight that protrudes out front (I'm not talking about fat but the weight of the child, placenta and additional fluid, like carrying a bowling ball strapped to your belly) and swelling in hands, legs and feet. Many of these things can be abated by having a bit of room to move and inactivity and confinement can exacerbate them.

It is not unusual to upgrade someone with a broken leg or arm because of lack of maneuverability. This is not much different.