Cassius Dio “As for the Jews, who had again increased so greatly that by reason of their multitude it would have been hard without raising a tumult to bar them from the city [Rome], he [Claudius] did not drive them out, but ordered them, while continuing their traditional mode of life, not to hold meetings.”

Either they were converting or they were having babies, both of which were possible since they were known to care for their children. 

Also, I was editing my comment as you replied to it.