FWIW, I've been doing ab/core work throughout, but not as intensely as prior to pregnancy. My favourite ab work has been the stability ball abs add-on on the Basic Step DVD. I've been doing the entire segment except for the roll-ups. Instead of the supermans on the ball, I've been just going on all-fours and alternating lifting my opposite arm and leg.
Another core workout that really works well during pregnancy are all the core segments on the High Step Circuit dvd. They're like Boodcamp Light. You have to use the mix and match feature to put them all together.
Push/Pull and SuperSets also have doable ab segments (slight modifications required). Any crunches that she does on the floor, I do on the stability ball. I don't do the bicycle maneover in SS, but just keep doing crunches at a 2/2 tempo.
With having a big belly, it's hard to feel like I'm actually working my abs, compared to before pregnancy, but I figure I've got to be doing some good, regardless, so I keep plugging on.
Hope this helps a bit,
Sandra