As the potential for maternal and fetal injury is significant because of the musculoskeletal and cardiovascular changes at this time, any exercise recommendations should err on the conservative side. The growth and development of a new life requires the interaction of many of the body's systems. Exercise requires these complex interactions as well. Because both exercise and pregnancies depend on common systems, they can interact with each other. For example, pregnancy changes the muscular and skeletal systems, which are also basic to the source of locomotion and balance of the individual.
Exercise produces heat which can also disturb the developing fetus.
The progression of physical activity is an important aspect of any exercise program. During pregnancy, the patient's energy levels tend to gradually fluctuate. During the first trimester, physical fitness will decrease. Some women will perceive that the same amount of work will require more effort than before pregnancy and they will adjust their efforts accordingly. Other women, who try to continue pre-pregnancy activity levels, will need instruction on decreasing their efforts.