I found this beautiful prayer at the end of an Akathist to the Mother of God. I’ve added it to the prayers and liturgics page.
Prayer for Unborn Infants
Remember, O Lord, Lover of Mankind the souls of Thy departed servants, infants who died accidentally in the wombs of Orthodox mothers from unknown cause, either from difficult birth, or from some carelessness and who therefore did not receive the Mystery of Holy Baptism. Baptize them, O Lord, in the sea of Thy compassions, and save them by Thine inexpressible grace. Amen.
Lost Innocents 