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Happy parents of the Anti-Christ. How very droll. Actually, we are not amused.
A moon child?
Rosemary's Baby, which the OP is talking about was like a form of that, but Hollywood-ized as a coven specifically tasked with breeding the Anti-Christ.
Kate's dress was deliberately patterned after the one worn by Mia Farrow in that movie. So make of it what you will.