Not much Spoiler below.
And, as this is 3rd time posting this, I hope someone explains if this gets erased again. Maybe, does this really need NSFW tag?
After the Arc 4's conclusion, waiting for Roswaal and Beatrice, Subaru and Emilia makes a chat. When he notices her reddened face,
[Subaru : Emilia-tan!? Your face is reddening fast! Are you okay?]
[Emilia : So... that... Subaru told me... you... love me?]
[Subaru : Yes. I told you I love you.]
[Emilia : Ah... I'm really happy but...]
Subaru felt anxiety because it was almost likely to end, 'let's stay best friend!'
[Subaru : But as I told you before, I'm waiting for Emilia-tan to look at me, and making efforts to it.]
[Emilia : Yes, I'm happy about that. But, even if you tell me like that, I don't understand what it means to love someone. Same as before in the carriage or crypt, I can't tell Subaru anything yet even you told me you love me. I think this is... very harsh...]
Subaru is relieved that it wasn't outright rejection.
[Emilia : But! We must have proper talk about baby inside me!]
...
...
...
[Subaru : What?]
[Emilia : I don't know if it's boy or girl, and we must love either way. But, I don't know anything about this and don't know what to do... So, I have to talk with 'father']
[Subaru : Wait.... wait... Emilia-tan? First take a deep breathe and calm down. I will do it with you. Ah, there's snow in good time.]
Rubbing snows on his face, Subaru continues discussion
[Subaru : Emilia-tan. Baby means you're pregnant, right?]
[Emilia : Y..Yes... It would be a scandal during Royal Selection, but, baby doesn't have any fault and must be happily born! I want my baby to be loved properly by parents.]
Subaru is certain that he never did 'thing' with Emilia. When he thought about Emilia doing it with others, his thought abruptly stops.
[Subaru : Emilia-tan... Baby doesn't fall from sky or grow from field, you know?]
[Emilia : But, man and woman mwah~ makes a baby, right?]
Subaru is dumbstruck. He thought current Emilia looks really strong as mother and would be advantageous for her if left unchecked. But, when he imagines about Emilia with imaginary pregnancy talking to non-existent baby, he discards the idea of leaving her like that.
[Emilia : Subaru, are you regretting about mwah~?]
[Subaru : Never did, and like to repeat it anytime!]
[Emilia : Y...Yes...]
Subaru immediately regret saying that, because it must meant to Emilia like he wants even more babies.
[Subaru : Damn you, Puck!]
Subaru finally decided to leave it to Ram and Frederica, while Emilia nagging him to decide child's name.
TL : To Puck's defence, this isn't directly his fault
This joke continues on the start of Arc 5, where younger brother of Julius, Joshua arrives on the Roswaal mansion as envoy of Anastasia.
[Subaru : This child is Beatrice. As Mimi told you, she's a child between me and Emilia-sama.]
[Joshua : !?]
[Emilia : Really, that's wrong, Subaru. Joshua is shocked. I mwah~ with Subaru, but mwah~ doesn't make baby. I have studied it.]
Narration implies Emilia's knowledge is stopped right at 'Kissing doesn't make baby', and other females on Emilia's side think it's too early for Emilia to know the full detail.
