The oxygen requirement increases by 20% while you are pregnant; in part due to the effects of the placental hormone, progesterone. To accommodate this increase there has to be an increase in the number of times you breathe, and how deep your breath is. You are also more sensitive to carbon dioxide in the air while you are pregnant, as a result you breathe more quickly.
The increase in ventilation gives you the sense of shortness of breath. This is why stairs are more difficult when you are pregnant.
Progesterone causes the release of relaxin, which is a hormone responsible for all ligaments -- pelvic ligaments, lower back, hips, knees, elbows, and even ligaments in your eyes. This hormone is responsible for the increase in flexibility as well as a potential cause of back pain.
Because you are more metabolic while pregnant it is very important to control your body temperature and hydration during exercise. Wear layered clothing when temperatures fluctuate and remove layers to stay cool.
Benefits of Exercise in Pregnancy
The length of labor (i.e., contractions) is 30% shorter in women who have a routine exercise program during pregnancy. This is believed to be due to the stronger abdominal muscles the fit women have. (It takes a lot of work to push that baby out!)
There is also a 16% reduction in the need for forceps and cesarean section if you are fit during your pregnancy.
Fit pregnant women have a lower incidence of high blood pressure and diabetes when compared to their non-fit counterparts. This is due to both exercise and the healthier diet that people who exercise tend to have.