When you are carrying twins, the demands on your body are different than when you are carrying a singleton. As a result, you must modify all exercise. Some doctors recommend women with twin pregnancies stop working out after 20-24 weeks because they are at full gestation size around 28 weeks and therefore at risk for preterm labor. And more than likely you should not do core if you are pregnant with twins, especially if diastasis recti is present.
ACOG’s patient education book Your Pregnancy and Childbirth suggests that women carrying multiples should refrain from aerobic exercise. Or anything that could cause preterm labor. But again do what your doctor says is best, which could be swimming or prenatal yoga ONLY.
So, if you are carrying twins, get special permission from your doctor. You may or may not have permission from your doctor to exercise while pregnant with twins. And if you are pregnant with more than 2 babies, DO NOT follow an exercise program.
Please review ACOG’s Absolute Contraindications and review with your OB.