"The worlds of the living and of the dead have collided, and the only way for people to successfully avoid ghosts – aka Beached Things, or BTs – is to strap a prenatal fetus to their chest which then detects the BTs through a device known as an Odradek Scanner. These fetuses, confined to a pod which is mounted on the front of Bridges operatives' uniforms, are known as Bridge Babies, or BBs.

"This pod keeps the BB alive until it is decommissioned, which usually happens after about a year of service. The pod is designed to replicate the environment from which the BB was taken: its mother's womb. Bridge Babies are the result of a pregnant woman becoming brain dead, which facilitates the unborn child's connection with the Beach where BTs reside. The stillmothers, as they're called, are kept in an ICU at Bridges headquarters in Capital Knot City, where data taken from their wombs are used to periodically update the conditions of the pod that the BB resides in."

https://screenrant.com/death-stranding-bridge-babies-alive-dead-bb-pods/