Transverse abdominal muscle (TVA) is the deepest of the abdominal muscles, it wraps around the abdomen between the lower ribs and top of the pelvis. It’s function  is to stabilise the low back and pelvis before movement of the arms and/or legs occurs. 

Excessive lengthening of your TVA due to pregnancy can cause lack of activation of your TVA. And you can’t strengthen the muscle you can’t activate that’s why before you start any core exercises you need to make sure you can activate your TVA first and if you can’t you need to practice and re-train your body to do it.

Below video explains how to active your transverse abdominal muscle. You can start practicing early post birth.It might be hard to feel any activation at the beginning but don’t get discouraged, practice makes the master 😉