you need to get pregnant from a fishkind or deep one and it needs to come out as human it will have a longer pregnancy duration
it looks the same but it has a longer birth timer its like a 50% chance if you roll the check for human instead of monster to get deepone but its usually easier to spot if on hell difficulty since the birth timer can reach high 40s i suggest doing the ol save then abort check if unsure if its deepone reload save if not try again since elise will tell you what the baby was if you use his abortion services