In almost every prenatal yoga class I teach, I dedicate at least a few minutes towards helping moms strengthen their transverse abdominal muscles. Here is a link to a page describing some of the breathing exercises we do in my class. Strengthening these muscles will help you push and can help with back pain and posture as well. A lot of back pain in pregnancy stems from the fact that your abdominal muscles are not pulling their fair share of the load when it comes to keeping your body upright. Your abdominal muscles are weakened by a diastasis recti and strengthening your transverse abs can give your core additional support. Often too much emphasis is placed on traditional crunches when it comes to abdominal work, which are great for your rectus abdominis muscles but do little for your transverse abs. Modified plank pose can be great for strengthening abs in early pregnancy, just make sure to hug in your transverse abs (of course, as with all exercise in pregnancy, if it doesn't feel right, don't do it).
I encourage women to focus on what their pelvic floor is doing as they contract their transverse abs (the natural tendency is for the pelvic floor to contract when our transverse abs are contracted as well as to contract with an exhale) and what that might mean for pushing. When you push, if you push on an exhale while contracting your transverse abs, it would be helpful to release (rather then contract) your pelvic floor. This is a bit counter-intuitive and, in my opinion, why some moms have trouble figuring out how to push. You can practice at home. Contract your transverse abs and hold for a count of thirty seconds (don't forget to breathe!) and notice what your pelvic floor is doing (it should be tightening) now focus on completely relaxing your pelvic floor while your transverse abs remain contracted (don't push, just focus on relaxing the pelvic floor, there will plenty of time to focus on pushing when baby comes).