Pregnant women experience tightening sensations in their abdomens at both early and later stages of pregnancy. This tightening is caused by the expansion of the uterus, as well as growth and tightening in surrounding ligaments, according to the Bump.
The uterus expands during pregnancy from the size of a grapefruit to the size of a watermelon, according to American Pregnancy Association. The ligaments around the uterus that connect it to the groin are called "round ligaments," and they are prone to pain and tightness throughout pregnancy, according to the Bump. Round ligament pain is not normally dangerous or indicative of a problem, though women carrying high risk pregnancies should consult their doctors as soon as they experience round ligament pain.