Walking, swimming and yoga are all advised in pregnancy, however don’t overstretch if you are not used to regular exercise, if possible a walk for 20-30 mins 4 times a week would be satisfactory. Do not partake in contact sports, racquet games, horse-riding, ski-ing, cycling or scuba-diving.
Also be careful in your work place, avoid chemicals, x-rays or heavy lifting. For backpain try using a gymball as it is often helpful and you can also use it in labour. Sit in a good chair well supported with cushions if need be. Hot showers are often helpful.