It's called a diastisis (diastisis recti to be precise) and DO NOT DO CRUNCHES OR SIT UPS. They won't help and in fact tend to pull the muscles apart even further. I have one, it was 2 fingers when Jazz was 6 weeks old and is now only half a finger (so was about 3cm and is now only 1cm max.). Do CORE MUSCLE exercises which will help strengthen the muscles, and like MR said pelvic floor exercises as well. And when you're getting out of bed lay on your side, legs over the bed, push up with your arms and sit up like that. If you sit upright it can pull the muscles to the side.

Defiantely do a physio class, maybe now and also once bub is born. My hospital had one running so it was free (YAY! Free! ), but a lot of private physio's will know how to help treat it as well.