Uploading this for commentary on the S episode due to it being one of the only logical explanations for what happened. From the doujin Platinum 78, available on lililicious.
(Michiru grabs baby Hotaru, stomping on Sailor Moon’s head)
Uranus (in shock): Neptune! Wh-what’re you
Neptune (head swiveling around with a mwahahaha): Uranus! Hurry up, let’s go!
Uranus: Wait a second! What are you planning to do with that baby?
Neptune: Raise it, of course! It’ll be our baby!! HOHOHOHOHO!
Uranus: I HATE THAT PART OF YOU! Are you some sort of demon…?
(Michiru zooms off, still cackling, Uranus going after her) WAIT NEPTUNE! I can’t keep up!
Usagi (stunned): Mamo-chan…
Narration: A friend of mine told me that’s how this scene went.
I like to think the stunned Usagi is muttering Mamoru’s name because Michiru’s baby loving jealousy reminds her of him exactly.
If you look at the manga, you can see that Michiru is actually pretty baby-crazy (she’s holding Hotaru in a LOT of panels)
And I love how Haruka is like “I HATE THAT PART OF YOU!”, clearly indicating that Michiru has done something like this before.