A weight-lifting routine will improve muscular strength and fitness, provide higher energy levels, reduce back pain, aid in labor and recovery, and control weight gain. If a weightlifting program was performed prior to pregnancy, then it is safe to continue your same routine until after the 1st trimester when exercises in the supine position (on your back) are not recommended. If you are beginning to lift for the first time, use machines instead of free weights to ensure safety. (If using free weights, be sure to bend knees to lift and lower weights to avoid strain on the back.)
Lifting low weight with high reps is best-focus on good form and proper breathing. Do not lift weights for the same muscles on consecutive days (i.e. if you lift upper body on Monday, rest one or two days before lifting upper body again). You could also combine your leg and arm workout into one session as long as you do not repeat the same exercises the next day. Avoid holding your breath, and concentrate on always exhaling on exertion. Due to the increased laxity in the joints, be careful not to overextend during exercise. The strength gains from weight lifting provide support to the joints, which assists in preventing injuries that occur due to increase in hormonal laxity.
Cardio & Aerobics
Any form of cardiovascular work is good for you, whether it is walking, running, stationary bike, aerobics, or swimming. The key is to pick what is comfortable to you and stick with it. Stay well-hydrated, avoid extreme temperatures, and steer clear of uneven terrains or surfaces, which may cause loss of balance. Wear supportive shoes and clothing. Abdominal supports can be worn to increase comfort in late pregnancy.
Monitor your intensity level using the Rate of Perceived Exertion (RPE). This scale ranges from 1 to 10, and is a self-assessment. When pregnant, you should exercise between level 4 (Somewhat hard) and level 6 (Hard).
Sample RPE Scale