Yep, I had it...was diagnosed when I was 31 weeks preggers and the physio could fit her fist into my separation

! I was gutted as I had always had very strong ab muscles and a flat, taut tummy (not anymore

)
The seperation has decreased to probably one finger now, but you can actually see it (which is pretty gross) as it sort of 'concaves' inwards

. I started my tummy tucks after 6 weeks and then moved onto some pilates excercises, but alas, it hasn't decreased anymore than that.
If you figure out a method to tighten those muscles even more, please let me know