Why do innie belly buttons pop out during pregnancy, but not weight gain?

The stretch pressure on the skin is different when it comes to weight gain and pregnancy. Pregnancy: The fetus is growing and there is an increase in amniotic fluid which cause the belly to quickly expend. Weight gain: Stretching of the skin is more moderate and less outwards push compared to the growing belly of a pregnant mum.

I have an innie and it never popped out at all. Maybe mine's a deep innie.

thanks