You can prevent your mid line from opening more, as well as speed your postpartum rehab by performing abdominal compressions (either standing, seated, or side-lying) daily. For more intensity, you can add on lower spine flexion to standing abdominal compressions.
For women with mid lines of 3 finger-widths or more, you can add manual splinting, with your hands, of the Rectus Abdominis, to further close the gap.
These exercises are safe to do right up to labor and delivery.
To view how to do these properly during pregnancy, go to:
http://www.youtube.com/watch?v=9cdSxWvF06g
http://www.youtube.com/watch?v=zkTlpSCmJyY
http://www.youtube.com/watch?v=juMlSPiZHTw
http://www.youtube.com/watch?v=bxbsq808lxE
BeFit-Mom