The First Omen is technically a prequel to Richard Donners 1976 classic The Omen, but its demonic pregnancy storyline is much closer to a different horror movie masterpiece. Set in 1971, The First Omen revolves around an American novitiate named Margaret Daino, whos sent to work at an orphanage in Rome and ends up uncovering a sinister religious conspiracy. As it turns out, shes the offspring of a jackal demon and shes been earmarked by a cult to become the mother of the Antichrist.
https://screenrant.com/the-first-omen-prequel-similar-rosemary-baby-horror/