Hi,
abdominal exercises in the first 12 weeks are apparently OK but once you get past that stage you need to be a bit more careful. Sit-ups or crunches when you are lying flat can not only play havoc with your blood pressure and make you feel dizzy but can as your uterus grows, its weight can compress the blood vessels leading to your heart, potentially depriving your developing baby of oxygen.
Water aerobics, fitness balls or, being on all fours and arching and dipping your body are all safer options. Hope this helps.