We have never invented anything special for this, or have we ever heard anything special for this, as far as the Compassion/Lovingkindness meditation, but by all means, feel free, give the woman a good wish and give the baby a good wish, too.
Since the child has not been born, it is fairly safe to assume that it does not have any ability to purify, but the wish can be modified a little bit. Instead of saying the wish as I and Rosemary normally give, "May they have peace of mind," we could change the beginning and say, "When this baby is born, may it find peace of mind." So just add that little saying, "When the baby is born…" Or you could also say, "If the baby is born," realizing the baby may die before birth.
Similarly we can work with people who have already died. Consider your grandparents, most of you have had your grandparents die, if some of you have them still alive, then consider your great-grandparents, and so on, you don't know where they are. Fair enough, we can consider that Buddhism teaches rebirth, but we don't know it for sure, so if you wish to do a Compassion/Lovingkindness wish for any relative or friend who has died, you can add a little beginning by saying, "If rebirth is true, may they find peace of mind." The exact same wish wherever they are, reborn human, in heaven realm, hell realm, wherever it is, it doesn't matter. Give them the exact same wish, it doesn't hurt.