Sorry to hear you are having back pains!! It's awful isn't it..

The first thing to do if you haven't already is to make sure you're wearing flats. One of the first symptoms of pregnancy for me was severe back pain to the point where I would be walking along and then not be able to move at all! Wearing flats stopped this from happening.

In terms of stretches, and being mindful of your bump of course! I would recommend the "cat-like" stretches. So this would be on all fours, gently stretch your spine upwards. The other one that is good, is while you're still on all fours, lean back onto your haunches and try to elongate your spine.

If you are still having problems, I would definitely suggest speaking to your midwife and asking for a referral to a physio.