During your first trimester, it's usually fine to continue working out your abs -- although between the morning sickness and fatigue you may be feeling now, it's natural that you may be less than motivated these days. However, once you're in the second trimester, these exercises are off-limits, since lying flat on your back can cause your blood pressure to drop, disrupting your baby's blood supply. And chances are, you won't be able to do them anyway once your belly starts to really pop. If you do crunches early in pregnancy, it's best to try a modified version that keeps your knees elevated. Compared to standard crunches or sit-ups, this position reduces your risk of developing diastasis, a condition where your abdominal muscles separate too much as your uterus expands into them. It also helps tone the muscles in your pelvic floor, which prevents back pain by providing extra support to your spine.