What exactly is a "uterine wall expansion" and when does that typically start happening in pregnancies?

Uterus in pregnancy: The uterus in pregnancy expands to accommodate the growing fetus. The uterus is made of smooth muscle, needed at the time of labor to contract and 'sweeze' the baby out. Much like a baloon expands because the amount of air increasing, so does the uterus (uterine walls) do the same. This begins in the first few weeks of pregnancy; within a month after conception, pelvic exam shows enlargement.