Doing sit-ups while you’re pregnant isn’t a good idea for two reasons.
First, lying on your back lowers your blood pressure, basically making your heart have to work harder to circulate blood because the weight of your uterus compresses major blood vessels. In the first trimester you may barely notice this effect, but as you get into the second half of pregnancy lying flat on your back may make you feel a little dizzy. It also makes the weight of the baby compress your stomach, which can be uncomfortable.
The other reason to avoid sit ups or crunches is that it can train your abs to go out and apart instead of in and together. Your rectus abdominis muscle is the one that gives you that six-pack look. Pregnancy pushes this muscle out and apart, and exercising your abs encourages this.
Sit-ups also aren’t the best for flattening a tummy after pregnancy– they condition the muscles in your upper abs, but it’s the lower abs that will need the most help after pregnancy. Exercises such as leg lifts, bicycle crunches and planks will be more effective for toning up the lower ab area and minimizing the “postpartum pooch” that all women get.