The belly, in Psalms 16:4, signifies the interior understanding.

In Genesis 25:23, it signifies conception of interior and exterior good. (Arcana Coelestia 3293)

In Isaiah 46:3, the belly signifies from the beginning of the church. (Arcana Coelestia 4918[2], 5550)

The belly signifies people who are being regenerated, and people who have been regenerated. (Apocalypse Explained 710[3])

(References: Apocalypse Explained 622)

3856. 'And He opened her womb' means that this affection gave rise to the doctrines taught by the Churches. This is clear from the meaning of 'opening the womb', or conceiving and bearing, as a Church coming into being. And since it comes into being through doctrinal teachings, 'opening the womb' also means the doctrines taught by the Churches. For conceptions and births in the Word mean spiritual conceptions and births, like those that take place when a person is born anew, see 1145, 1255, 1330, 2584. The implications of these matters will be evident from what follows shortly.