What can I do?

I am 23 wks. pregnant with my third baby and I work out regularly.. I joined a kickboxing cl a few weeks ago but there are some exercises I can't do, like abdominal, those really being the main ones.. What could I do while they are working their flat tummys?lol.. I usually will do girly push-ups, or the piltaes see-saw, do you know of anything i could do? even exercises to work my adominal muscles but of course not lying on my back..i know its safe to work them out cause thats where your whole prengnancy revolves around but it's not safe to lie on your back..if anybody has suggestions, then let me know..thanks!!