As the uterus expands during pregnancy, the muscles of the abdomen (rectus abdominus; commonly 'six pack muscles' and yes, even it you can't see them, we all have them!) can be stretched and separated. This isn't a gap as such, just a stretching and thinning of the connective tissue between the muscles.
Almost all ladies will experience this, so know that it is completely normal. It will take around 6-8 weeks for your body to start the recovery process naturally so there is no urgency to check in the very early days post partum. Look out for bulging or doming in your midline and focus in the early on diaphragmatic breathing and pelvic floor (see previous post on post partum 'fitness'.)
If you're in a position to get a check by a pelvic health physio, I'd highly recommend this. If not, check using the method below if you feel comfortable.
1. Lie flat on your back on a flat, firm surface with your knees bent.
2. Lift your head and shoulders off the floor to a partial crunch position.
3. Feel along the linea alba (connective tissue that separates your six pack muscles) from your chest bone to your pubic bone checking for any areas of softness. In pregnancy, the largest part of the bump is normally around your belly button so it may be that you feel more softness here.
4. Don't worry too much about the size of the gap, just feel for areas of tension.
I would always recommend you see a pelvic health physio or post partum exercise specialist before returning to exercise or look out for the next post on some simple exercises you can do at home to support your diastasis recti.