It depends. I have two explanations :
1) They had sex together and God impregnated her.
2) Both are virgins and God isn't the father of Jesus. Jesus is a teratoma from Mary.
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Alum dare, dolere, id Hephaestus, id ire / Pro profundis fati / Pro pulchris infernarum profundis / Pro pulchris omni fati brachium / Pulchris profundis infernarum servi fati / Profundis, profundis fati