Different areas and systems are under increased demand for performance, energy, circulation and growth, while others take on supporting and preparatory roles, as you prepare for labor and delivery, enabling the growth and development of the fetus and staying healthy yourself.
Pregnancy is not an illness, but there are discomforts and realities to consider when you do opt to workout and stick to your daily, weekly exercise routines, workouts and classes (treadmill, stationary bikes, weights, abs-machines, high-intensity aerobics might not be in the cards for you, but with modifications and adjustments you can still get a full body-workout, work on target areas, like lower torso, abs, stomach, pelvic and hip muscles. They are not off-limits, will just be dealt with differently. Allowing some extra time to breathe, get oxygen-rich blood and air circulating throughout your body (it might take you a little longer to get ready and cool down at the beginning and end of your workout).
Your target heart-rate zone, might vary and be changed to accommodate your pregnant body as well. This is normal, standard and prudent to workout safely, risk-free, throughout the different stages of your pregnancy. Pure water, flavored (less to no sports-drinks), fruit-juice like orange, apple, tropical smoothies, after workouts can go along way to keep you hydrated, boost circulation and cool you down, quench your thirst and replenish your body.
The more stretching and warming up your do for/of your muscles, the better as they are under pressure to deliver more literally speaking, so using your warm-ups to work out safely can also help you down the road, in labor and delivery, capture and embrace the opportunity to flex, prepare and stretch those muscles groups and target areas one and all.