Most exercises are safe to perform during pregnancy, as long as you exercise with caution and do not overdo it. Keep in mind that pregnancy is a time to maintain fitness.
The safest and most productive activities are:
indoor stationary cycling
step or elliptical machines
and low-impact aerobics (taught by a certified aerobics instructor)
These activities carry little risk of injury, benefit your entire body, and can be continued until birth.
Tennis and racquetball are generally safe activities, but changes in balance during pregnancy may affect rapid movements. Other activities such as jogging can be done in moderation, especially if you were doing them before your pregnancy. You may want to choose exercises or activities that do not require great balance or coordination, especially later in pregnancy.
Whatever exercises you choose to do throughout your pregnancy, be sure to check with your doctor first!