A Bridge baby, also known simply as a BB, is a baby that has been taken from a stillmother to be used by Bridges operatives. A stillmother's womb facilitates a connection between the world of the dead and a Bridge baby, granting Bridges operatives who establish a trance connection with a Bridge baby the ability to sense BTs. Most babies are retired after a year of service.
Bridge babies are kept in portable pods which simulate the conditions of a stillmother's womb, as they will cease to function properly if they feel they are not in such a womb. Periodically, the environmental conditions of a Bridge baby's pod must be updated in a synchronization incubator with data gathered from a stillmother's womb at a given moment.
Source: Death Stranding FANDOM Wiki, "Bridge Baby", available under the CC-BY-SA License.