For a women with no pregnancy complications, any exercise done in moderate levels is considered safe during pregnancy. It is also advised not to take up any difficult exercise (especially if you are new to it) while pregnant.
Women with medical problems (like asthma, heart disease, uncontrolled type 1 diabetes), and obstetric condition (like Bleeding/Spotting, Weak Cervix) are advised not do exercise.
Safe Exercises During Pregnancy: Most exercises are considered safe during pregnancy, as long as you do not over do it. Exercises like Swimming, Walking, Bicycling (indoor) falls under this category. Jogging and running can be done in moderation.
According to WebMD, the following should be avoided during pregnancy.
Holding your breath during any activity
Activities where falling is likely (such as skiing and horseback riding)
Contact sports such as softball, football, basketball and volleyball
Any exercise that may cause even mild abdominal trauma, including activities that include jarring motions or rapid changes in direction
Activities that require extensive jumping, hopping, skipping, or bouncing
Deep knee bends, full sit-ups, double leg raises and straight-leg toe touches
Bouncing while stretching
Exercises that require lying on your back or right side for more than three minutes. (especially after your third month of pregnancy)
Heavy exercise spurts followed by long periods of no activity
Exercise in hot, humid weather
If a pregnant woman is new to exercise, doctors generally advice to include walking in their daily routine, as it is safe to perform in any trimester.
Consulting healthcare provider before starting any exercise is always better for a pregnant women and her developing baby, as he/she, considering your past medical history, provides necessary suggestions and tips for safe exercise.