​The most significant muscle stretching in the core happens along the rectus abdominus and impacts the connective tissue called the linea alba. The linea alba runs along the midline of the rectus abdominous from the sternum to the pubic bone connecting the muscle bellies (think of the muscles we see in six-pack abs) of the abdomen. As the uterus expands, the muscle bellies can separate and the linea alba stretches thin.  This creates what is called a diastasis recti.  In its most scientific sense, diastasis is a musculoskeletal injury, where the rectus abdominus tears at the connective tissue, separating it from the linea alba.