Safe Abdominal Exercises...?

Eva
Any tips on safe abdominal workouts during pregnancy? I'm currently 15 weeks pregnant. I've been told it's not as safe at this point to do sit-ups or lay on your back. 
What has worked for some of you..?? 
**Please note I'm just trying to find a simple workout plan that keeps me and the baby healthy, and prevents me from gaining unnecessary body weight. 
Thanks!