itsallgoingtobefine your rectus abdominus muscles separate (the ones that go across your stomach in horizontal bands and give you the horizontal seam down your middle where they join). They do this to allow room for the expanding uterus. Once you've given birth these muscles slowly move back into their previous position - taking around six weeks after a vaginal delivery and up to 12 weeks after a caesarean delivery.
However, sometimes the connective tissue between the, the linea alba, overstretches or tears, which causes a split called a disastasis recti. This means the rectus abdominus don't move back into their previous position or do so only partially due to the tear/stretching of the linea alba. The separation can vary - 2 fingers or less of separation is classed as 'normal' but people can have up to 9 fingers of separation. Separation can be gauged at the 6 weeks postnatal check by doing a simple stomach curl and feeling for a gap between the abdominals. A separation means that core strength is diminished. Weak stomach muscles put pressure on the lower back and also the pelvic floor. They also cause 'pouching' of the abdomen as the abdominal organs are not held flat by the abdominal muscles.
There's nothing you can do to prevent it occuring during pregnancy (although doing abdominal exercises during pregnancy is thought to increase the chance of it happening and also the degree of separation). THere's also nothing you can do during pregnancy to treat it so there really is no point in worrying about it. The good news is that you can close it with specific exercises. Google the Tupler Technique as this will tell you the exercises you need to do and also, key issue here, the exercises you need to avoid. People make the mistake of thinking standard abdominals (crunches and obliques, etc.) can resolve the issue as they're working the abdominal muscle groups but they can often fix the separation and sometimes make it worse.
I had a separation following the birth of my twins. Not massive (around 3 fingers or so) but it bothered me as I was a) vain, b) a runner and c) did lots of pilates. I wanted to close it and I did. I followed the Tupler technique for a couple of months and closed the gap to around 1cm or less, which is pretty good. I ended up with my six pack again. I'm currently 7 months pregnant and can see that I've got a separation again (not suprising given that my linea alba has torn/stretched from the previous time and it doesn't repair itself - the exercises just encourage the muscles to move as close together as possible). I'll be doing my Tupler exercises again.
Honestly, it will be fine. The gap is closeable with the right exercises and they're pretty easy to do. In fact, my abdominals ended up bloody strong from doing them so you may even end up in better nick afterwards.
Don't panic!