What does the baby Rachel have to get sedated?

The baby is sedated to keep it from crying and alerting the soldiers to their location.

Peter's voice was firm. "We can't take a chance," he said. He inserted the dropper of the bottle into the baby's tiny mouth,and squeezed a few drops of liquid onto her tongue. The baby yawned, and swallowed. The mother closed her eyes; her husband gripped her shoulder.