do you have a back problem? if so, you might as whoever is treating you (physician, therapist) about ab exercises.

I ask because anything that works your abs will also work your back because of the stabilization.

if there is just some reason that you cannot lie down but have no exercise restrictions, then here are a couple of ideas.

1. there are the crunch machines at the gym that are "backless"
2. cable crunches - sitting or kneeling at the high cable, and then crunch forward
3. sit-backs: sit on floor or bench in "ending position" of a sit-up (knees bent, arms around knees) and *sit back* to about 45 degrees and then back up.
4. sit on an exercise ball and bounce, or work your arms (curls, lateral raises, front raises, etc) - the effort of stabilizing on the ball will work your trunk.

if you want to work obliques rather than rectus abdominus, you can add:
1. full contact twists
2. side presses with db
3. cable "twists"
4. forget the name, but basically you place a db on the outside of your left foot. bend over and grab with your right hand - full twisting stretch as you bend - and then uncoil and straighten up as you raise the weight.


etc.

----
I just re-read your post and noticed the goal about being pregnant - if you are pregnant - I've seen a lot of advice about not working abs specifically at all while pregnant - except for the bouncing on the swiss ball. check with your OB about ab exercises if pregnant.