When in pregnancy does your lower stomach or abdomen begin to harden?

During the second trimester of pregnancy, usually between weeks 12 and 16, women may start experiencing a hardening of the lower stomach or abdomen. This is due to the growth of the uterus, which is a muscular organ that stretches and expands to accommodate the growing baby. As the uterus grows, it can put pressure on the abdominal muscles, causing them to tighten and harden. This hardening of the abdomen is known as uterine contractions, or Braxton Hicks contractions. These contractions are usually irregular and painless, and are the body's way of preparing for labor.