fredman,

Baptism with water in the name of the Holy Trinity is always efficacious because it is God's work alone not an act of man. It is beneficial when received by faith alone.

John the Baptist was regenerated and given faith in the womb by the outward word of Mary. This is proven because joy is a fruit that follows faith. If an unborn child in the womb can be given faith, then surely a child can receive faith through the outward word of Baptism.

Lutherans don't keep a scoreboard of conversions based on baptisms or preaching. We are commanded to baptize and to preach. We know baptism and preaching are efficacious. That is enough.