Limbo has never been an official teaching of the Church. Medieval theologians tried to explain what happened to infants who died without baptism. The Lord said that baptism is necessary for salvation, for heaven (John 3:5), but an infant could not be guilty of any personal sins. So they speculated that these children would go to Limbo.
Today, Limbo is not mentioned in the Catechism of the Catholic Church. We remember the Lord’s tenderness towards children (Mark 10:14) and entrust those who have died to his great love and mercy. It is still Catholic practice that infants be baptised ASAP after birth.