Regular gym work is fine while you are TTC. When you are pregnant there are lots of benefits to staying fit. AFAIK there are no problems with working on your abs until after you're pregnant. When you are pregnant they recomend that you don't work out past the level at which you can continue to talk IYKWIM so maybe you could stick to that whilst TTC. I didn't back off my gym work until after I was pregnant. (that was with number 1 - I've barely seen the inside of the gym since then ). Once you're pregnant you need to be careful with stretching because of a hormone called relaxin which makes it easier to stretch too far.
It might even help with TTC because just so long as you keep up your fluid intake excercise can increase your production of CM.