The minute you get pregnant the hormone relaxin kicks in which prepares your pelvis for childbirth. This relaxes all your joints throughout pregnancy and will stay in you until you stop breast feeding - though it obviously lessens over time.
So, avoid sudden twists & jerks, avoid over stretching, avoid running post natally until you are at least 14 weeks PN (& not at all if you have any pelvic floor issues), avoid all types of crunches/sit ups & try and roll to your side to get up and lie down. If you have a slight or a large gap in your abs (diastasis) then you really must avoid putting more pressure on the abdominal wall and pelvic floor, so that is why you do not want to start doing abs exercises involving the plank or even 1/2 plank, the rectus abdominus (6 pack abs!) or the obliques (side abs) as they can make your tummy more domed or increase the gap you have.
So it is all about the deep diaphragmatic breathing & as soon as you can start learning to breath properly. I use the balloon as an example - fill it with air / fill your lungs/tummy with some air & then let the balloon go / let your abs go as you release the air and let the tummy gently go in towards the spine. Practice, practice & practice this as this is the way to get better & flatter abs!!