The reproductive suppression field, also referred to as the suppressing field, was imposed by the Combine upon gaining dominance of Earth. It prevents human procreation as part of the Combine plan for population control.
The suppression field's main effects are to bring the human birth rate down to zero by prohibiting the formation of "certain protein chains important to the process of embryonic development," as Dr. Kleiner states in Episode One, making it impossible for conception to occur in any form. This effectively ensures that the humans currently alive will be the last generation to survive. As a result, no children can be found on Earth by the time of Gordon's arrival after two decades in his prolonged stasis following the Black Mesa Incident.
The suppression field, as later mentioned in Episode One, is supposedly functioning within, or at least depends on, the Citadel of City 17. It is never mentioned, however, where exactly the installation is located. When the Citadel is severely damaged by the end of Half-Life 2, the Supression field is shutdown. 5 years before the events of Half-Life 2, when the Citadel was being built, the supression field is never mentioned. It is unknown if it was already functional at that time or if it only turned on with the fully operational Citadel.
