Can I do crunches/situps while pregnant?

1st trimester only. After the 2nd trimester of pregnancy, the rising abdominal uterus will make the situp task more difficult. Moreover, the major blood vessel bringing blood back to the heart(inferior vena cava) also gets "crunched".- not a good idea for the baby. There are several great aerobic exercise classes for pregnant mothers that keep you fit and are safe for the developing baby.