Sit-ups are generally not recommended during pregnancy. This is because the movements involved may traumatize the uterus or abdomen.
Sit-ups are of particular concern after about the 4th month of pregnancy. Any time you are on your back after this point, you run the risk of the uterus compressing the vena cava (a major blood vessel) which in turn would reduce your blood pressure. Generally, any exercise that must be done in the "supine" (on your back) position is not recommended during pregnancy.
You can, however, still perform exercises while you are pregnant, including abdominal exercises. One of the best abdominal exercises you can do is "Mini Sit-ups." Mini Sit-ups will help prepare you for labor by toning your abs. During the second stage of labor your abs will do much of the pushing work. Additionally, your abs provide support for your spine and may help prevent low backache.
To perform Mini Sit-ups, follow these steps:
1. Lie on your back, knees bent, with your feet on the floor.
2. Place a pillow under one hip so you’re not flat on your back.
3. As you exhale, tighten your abs and raise your head and shoulders off the floor while reaching for your knees with your arms.
4. Inhale and return to starting position.
5. Repeat 10 times, once in the morning and once in the evening.