From the website of the United States Conference of Catholic Bishops:
Following Vatican approval, the “Rite for the Blessing of a Child in the Womb” is now available for use by dioceses in the United States. The text of the blessing is posted online and is being published as a booklet addendum to the Book of Blessings/Bendicional. The blessing will be included in future editions of those liturgical volumes.
“We hope the use of this blessing will provide not only support and God’s blessing for expectant parents and their child in the womb, but also another effective witness to the sanctity of human life from the first moment of conception,” said Archbishop Gregory M. Aymond, chairman of the Committee on Divine Worship of the U.S. Conference of Catholic Bishops (USCCB).
The blessing can be offered within the context of the Mass as well as outside of Mass.
The text is available here.