I get this mainly when I’m trying to lay in bath, I used to panic over it a lot but she’s healthy just don’t do it too often if possible as it can be chasing a split in abdominal muscles after birth but if your concerned check with your midwife x
Its stomach seperation called Separated stomach muscles It's common for the 2 muscles that run down the middle of your stomach to separate during pregnancy. This is called diastasis recti, or divarication. The amount of separation can vary. It happens because your growing womb (uterus) pushes the muscles apart, making them longer and weaker. You can get referred for physio by your MW
I think this is called coning when your stomach muscles do that and you should avoid it where possible as can cause complications after birth with your abdominal muscles splitting. Don't think it's anything to do with the placenta but I could be wrong! If you Google coning it should give you more info xxx