I was going to ask you what your thoughts were regarding crunches. The classes I do all have a crunch portion, and what I've read says that you shouldn't do them on your back after the 4th month... supposedly it can reduce oxygen flow to the baby... I'm just wondering how much it actually does this. I've been doing the crunches up until now, and I can always modify a bench so that I'm on an angle, but I would rather not. I don't want to hurt the baby either.
Great question!
I've been modifying various ab exercises, especially since I'm still teaching a step-n-sculpt class, and there would be a riot if I didn't make them work abs, too!
The important things are that 1 - you don't do anything new, that you haven't done before you got pregnant, 2 - you feel ok while doing it (no pulling or cramping), and 3 - you NEVER hold your breath. From now on, think of abs as a yoga workout, with a constant and controlled inhale/exhale.
PLANK:
This should be done from the knees only - the low back is getting weaker. Challenge yourself by keeping your shoulders behind your elbows, and really dropping your hips/tucking in the butt.
SIDE PLANK:
Keep one knee down, and always press the hips forward to maintain a straight body line.
CRUNCHES:
It should be fine to do crunches from
an inclined position if your body says it's ok. You can also try crunching on a ball - but keep your feet wide for balance. Otherwise, I've been modifying crunches by sitting up and crunching back; drop from a fully sitting-up posture to a slightly slouched posture... see stick figures, below. You can add a twist at the top, too. 
LOW BACK EXTENSIONS: NO WAY.
This is mostly because lying on the tummy is probably getting more and more uncomfortable -- and I can't help but picture a kid's face smooshed against a window... But, since the low back needs to stay strong and flexible as your centre of gravity changes, I like the cat/cow option. Start on your hands and knees and exhale, tucking your chin and pelvis under, and pushing your back as high as possible, pulling your abs in. Then inhale, and reverse it: chin and hips lift up as high as you can, and the low back/abs drop down low. Think about exaggerating each posture as much as possible, and don't forget to breathe deeply. (see stick figures, below)
KEGELS:
Gotta do em. Lots! Often! Daily!
All of these are good options, BUT - make sure your doctor approves!!! If you have the diastasis rectus abdominus (common condition in which the abs separate), you may be advised not to do any abdominal training, in which case working your transverse abs by holding in your stomach may be all you can do.
PERSONALLY, I'm starting to lean towards "why bother" for my own abs, although I'm finding the cat/cow is excellent for stretching things out and making me feel stronger.
Hope this helps! (but check with your doctor!)
