Answer:
Ask your parents. Follow what you believe if you're really faithful to it. If not I'd say do what you want.
Birth defects can be caused wither by internal genetic factors or external factors like drugs, radiation, smoking,...
Dana, who is concerned about the possibility that her baby will have defects, may do the following tests: screening test, that will show if she might have child with birth defect and diagnostic test like amniocentesis (analyzing small amount of amniotic fluid from the area surrounding the baby) which is more than 90% correct.
The minimum amount of aerobic activity that one should engage in is 30
minutes 3 times a week, less than 30 minutes will not be effective.
