Hi Susannah,
I'm trying to think back to the pregnancy - it was only a few months ago for heaven's sake, and it already feels like years ago. I had an unsual situation whereby I was put on bedrest for 9 weeks, at the start of my second trimester. Prior to the bedrest, I didn't modify the impact level of any of my workouts. When the doctor gave me the okay to exercise again, I was halfway through the pregnancy, and I started off really slow and easy. Within a few weeks I had worked my way back up to a good intensity level, but can't say that I ever returned to the level I was at prior to the bedrest. Essentially, I just did my best to listen to my body, and decided to modify based on what felt comfortable. That's going to be different for everyone, and something you need to gauge for yourself.
What I did not do was simply stop doing a particular exercise just because some book told me not to. For example, I read in many places that pregnant women shouldn't do kickboxing or step aerobics because of a lack of balance due to the growing belly. While I'm sure many women do find their balance gets a bit off as the pregnancy progresses, I didn't find that to be the case for me, and I kept up with all my kickbox and stepping workouts. It's all about personal judgement, IMO, and rarely do general rules apply to everyone.
Regarding the ab work, it is suggested that after the 12th week women don't lay flat on their backs, as the heavy uterus blocks of the blood flow going through a major vein to the heart. With both my pregnancies I found that laying on my back after about the 12th week made me feel nauseas and dizzy, so I modified my ab work starting at that point. One recommendation for pregnant women is to do crunches on a bit of an incline. So, I modified my ab work to an incline on the stability ball, starting around the 12th week. I stopped doing various other ab work (such as the Intensity Series) over time, when the moves just began to feel uncomfortable to do. So that happened kind of gradually.
I forgot to mention that the core work in High Step Circuit (the beginner/intermediate workout) was great for pregnancy, as was the core work in SuperSets (modifying the bicycling to crunches on the ball).
Hope this helps!
Sandra