There are blessings for adoptions, and adopted kids. Infant baptism, too, of course. It makes sense to add one for a child still in the womb.
Any good priest, if approached, would surely be happy or have been happy to compose a blessing. Lots of guys would just do it on the spot, right?
Having an official one for inclusion on the Book of Blessings is a nice thing. But just because there hasn’t been an official one before doesn’t mean it hasn’t already been done. Thousands of times.
Good Scripture, Psalm 139.13-15:
You formed my inmost being; you knit me in my mother’s womb. I praise you, so wonderfully you made me; wonderful are your works! My very self you knew; my bones were not hidden from you, When I was being made in secret, fashioned as in the depths of the earth.
Dan Schutte was on to this good psalm a generation ago when he wrote “You Are Near.” Psalm 139 doesn’t appear terribly often in the Lectionary. But maybe it should. It’s a great act of worship and a rich prayer with many blessings. Including blessings for the unborn blessings.