The Catechism in Questions & Answers

684. What is the prenatal blessing?

The prenatal blessing is the first visible act of God upon an as yet unborn human being. This blessing serves to benefit the unborn soul, and is performed upon the mother-to-be. Through the prenatal blessing, God strengthens the mother in her endeavour to promote and cultivate the faith life of her child during its prenatal development.
The prenatal blessing is associated with God’s help during the time of pregnancy and for the birth of the child. This does not imply the promise of a problem-free pregnancy or the birth of a healthy child, however.