In his commentary on the Gospel of Luke, St. Ambrose mentions the following verse:
“Behold, children are a heritage from the Lord,Psalms 127:3
The fruit of the womb is a reward.”
He explains the deep communion between Jesus Christ and the faithful.
As Christ lives in us, he creates a clean heart. His work brings a flower then a fruit. It takes work and time. The fruits are signs of life. At the end, if you have no real relationship with Christ, you have no life, you cannot bear fruit.
Gambero, L., 1999. Mary and the Fathers of the Church 1st ed. Translated by Buffer, T., San Francisco: Ignatius Press.