Some of the benefits of exercise for pre postnatal fitness training clients is more energy during pregnancy, reduced stress & anxiety, quicker recovery from birth and fewer complications. Also keeping fit while pregnant has been proven to reduce labor time and boosts self esteem!
During a pregnancy period a lot of stress is put on the muscle on the lower back due to the heavy weight being carried. Exercise and stretching can be extremely important to increase strength and reinforce the body structure for the extra weight.
During the second trimester (weeks 13-28) the unpleasant symptoms from the first trimester should have settled down and the mother will start to feel a renewed level of energy. The biggest change during this period will be the growth of the ‘bump’. As the baby grows the mother should notice a change in her body position and posture. The extra weight occurs at the front of the body, the hip and back joints will take the strain. The posture will shift as the lower back muscles tighten and take an exaggerated curvature. The abdominal muscles will stretch and strain to adapt to the growing baby. During this time it will be extremely beneficial to perform exercises to help strengthen weakened abdominal and pelvic floor muscles, in order to reduce the strain on the lower back.
Source // Wikipedia