There are many profound changes with the respiratory system in women during pregnancy week by week.
The adaptations are controlled primarily by progesterone and take place in the early stages of pregnancy starting soon after you miss your period.
The vital capacity remains normal. There is an increase in tidal volume (almost 50%), which causes an increase in minute ventilation throughout pregnancy (21% and 50% in the second and third trimesters respectively). The enlarged uterus occurring later in gestation increases the pressure on the diaphragm during pregnancy causing an increase in resting oxygen requirements and an increase in the work required for breathing. This happens because the diaphragm is a key muscle responsible for proper respiration, it helps to inflate the lungs; with extra pressure from the uterus it is harder for the diaphragm to contract, using more energy to bring in the same amount of air. This means there is a decreased amount of oxygen available for performance during aerobic exercise.
These respiratory body changes are important to understand while starting an exercise protocol during pregnancy. With the increased resting oxygen requirements, which will increase throughout term, it is harder to perform at the same level pre-gestation. Towards the end of term, one may not be able to maintain the same amount of intensity or work level as during the start of gestation.