What do you suggest if my sister is trying to do sit ups while pregnant can that harm the baby?

Good for her!: It is always encouraging to have graduated exercise programs for pregnant ,mom's as it can shorten and ease eventual labor. The rising uterus after the first trimester will minimize your sister's ability to do sit ups very soon. There are other types of exercises that will not compromise her or her fetus' wellbeing that involve cardio and stretching. The fetus is enclosed in a cushioned water sac.