Christmas Abbott is a successful and beloved CrossFit Athlete, Coach/Trainer and Gym owner of "CrossFit Invoke" in Raleigh, NC. In October 2018, she gave birth to her son, Loyal Atticus. Before and after that, she was in the gym, keeping herself active and healthy in ways that were safe for her and her baby.
We know of no better single exercise than the squat. It could be a body weight squat (as Christmas is doing in the "after" photo pushing baby Atticus). It could be a dumbell squat (as she is doing while 9 months pregnant in the "before" photo). Back squats, front squats, overhead squats, wall balls - they all work. Our quadriceps (thigh muscles) are the largest muscles in our legs, and our gluteus maximus is also pretty big. So it makes sense that working these big muscle groups would have big payoffs. Squats also recruit hamstrings, the 3 muscles at the back of the thigh that impact hip and knee mobility.
See more CrossFit bef-afters via the first two links, and see more about Christmas Abbott via links 3 and 4 to her Instagram and the CrossFit Invoke website.