A Bridge Baby, abbreviated as BB, is an unborn fetus that has been taken from a stillmother to be used as equipment by Bridges operatives, and some separatist groups, granting them the ability to sense and detect the Beached Things.
A Bridge Baby is a seven months fetus removed via a cesarean section from a brain-dead mother. A stillmother's womb facilitates a connection between the world of the dead and a BB, granting the operatives the ability to sense and detect BTs when physically connected. To function properly, 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. The majority of the BBs are withdrawn from service just after one year, apparently the maximum time they can be fooled into believing that the capsule they are in is actually the womb, before it becomes dangerous to the operative. Periodically, the environmental conditions of the capsule must be updated by inserting it in a special incubator that synchronizes it with the stillmother's womb at a given moment, taking the data in order to keep the deception longer.
Bridge Babies use a sensory mechanism similar to echolocation to detect BTs. The pod, designed to mimic the conditions inside a human womb, are key to preserving the BBs connections with this world and the other side. Bridge Babies are capable of producing something not unlike a dolphin's sound waves, travelling through the amniotic fluid and out the other side, bounce off BTs, and return as echoes, allowing a BB to ascertain the location of threats.
Occasionally, when Sam finishes his preparations in his Private Room and goes out into the outside world, Sam connects to his BB and experiences a flashback. Deadman likens the phenomena to "feedback, like a speaker" and warns Sam that "[connecting] risks taking you to a place that you can't return from". The use of BBs by the DOOMS-afflicted is generally prohibited for this reason.
The first recorded mention of a BB-like fetus and BTs relates to a caesarean section where a surgeon specializing in caesarean sections tried to save a seventh month fetus suffering from bradycardia from a brain-dead woman's body, placed on life support. With the incision made without incident, the surgeon planned to remove the fetus from the womb and immediately place it in a NICU, and for this he had to cut the umbilical cord. When the surgeon touched the umbilical cord - which connected the baby to the world of the dead - for the first time a human layed eyes on a BT, the following contact the very first modern Voidout.
The BB experiments begin
At the time, not knowing the cause of the explosion, someone hypothesized that by recreating the same conditions (brain-dead mother, fetus and umbilical cord) it would be possible to identify the BTs and the mystery of the Death Stranding would be solved, the government called the fetuses they used “Bridge Babies", and the experiments became known as the “BB Experiments".
The BB experiments were conducted in a Manhattan government facility in total secrecy until a massive voidout destroyed the island, killing the then-acting President who was witnessing the experiment. Vice-president Bridget Strand then became the new President and shortly after her inauguration she put an end to the experiments and had all data destroyed.
The experiments continue
However, Bridget believed that BBs could be exploited to prove the existence of BTs, revealing them to people and thus reassembling America. The experiments continued in secrecy more carefully under Bridges' supervision. BBs were placed into a specialized pod, and scientists understood that BBs were not only a way to make BTs visible, but also the power to connect the living with the world of the dead and the Beach.
However, not all BBs were created equal, and for the new network imagined by Bridget to reconnect America, a BB with unimaginable potential was needed, and that BB was the son of Clifford Unger and his wife Lisa Bridges. Clifford agreed to let the scientist use his son to research BB tech, in hope for a cure for his brain-dead wife, but was lied to by President Strand about the conditions and consequences. Soon after learning the President's intentions, Clifford attempted to flee with the baby but was shot during the escape. After being found and confronted, Clifford was fatally shot by John McClane and President Strand, accidentally killing the BB in the process. Bridget, however, brought the baby back from the Beach using Amelie, which caused a massive BT invasion with following Voidouts occurring worldwide and ultimately set the Death Stranding in motion.
After all this, information about the BB experiments were made top secret and was mostly expunged from the Chiral Network, leaving the people with the belief that BBs are only a tool for detecting BTs, and most Bridges employees without knowledge of how they work at all.
- Connecting to a BB seems to agitate a chiral allergy even in those not afflicted with DOOMS, as Igor Frank experiences allergic tears when he first connects to his BB.
- Higgs carries a portable tank with a fetus-shaped figure inside. Rather than emitting an orange-yellow glow when activated, as the Bridge Baby's tank does, his portable tank emits a red glow instead. The bottom casing of his tank is also green, rather than orange like the Bridge Baby's.
- BB-28 is the only active BB seen during Sam's westward expedition. Since the lifespan of the BBs is only one year, it is likely that the precedents have been terminated.
- The only other BB seen is the "Network Activation Key", a BBs used to complete the Chiral Network activating the last terminal in Edge Knot City.
- It is to be assumed that each Knot is connected to the network via a "sacrificial BB".
- The idea of Bridge Babies being used as living sacrifices is reminiscent of some folklorist fables surrounding the London Bridge, particularly that of live burials within the bridge's foundations. This is also referenced in-game in a particular interview.
- Hideo Kojima has stated that only one individual baby had been shown up until the TGS 2018 teaser trailer, and that it relates to the game's mechanics and story as a whole.