Most doctors recommend that pregnant women avoid weight training and sit-ups after the first trimester, especially women who are at risk for preterm labor.
Lifting reduces the blood flow to the kidneys and uterus, and exercises done on your back (including sit-ups and leg lifts) cause your heart rate to drop, also decreasing the flow of oxygenated blood to your body and the baby. It's better to tone your abdominal muscles while on all fours, by relaxing and then tightening your muscles as you exhale.
Unless your doctor tells you otherwise, it's also a good idea to avoid any activities that include:
- bouncing
- jarring (anything that would cause a lot of up and down movement)
- leaping
- a sudden change of direction
- a risk of abdominal injury