I was lucky enough to stumble across an article about it when I was pregnant so I knew to check for it once DD was born. I have roughly a 3-4 finger gap which means that sit ups are right out for me and instead of making my stomach flatter, it will make my abdominal gap worse.
A lot of women do get DR but aren't aware of it and as soon as they are able to, they start doing sit ups and aren't aware that it makes things worse.
If you want to check for DR, there are lots of tutorials about it on youtube but I just wish that it was part of antenatal / postnatal care so that women are better informed about it.