@Sine Wave  
@kztxl7  
Sil stated a few times here and there that they’re not immortal, just VERY long lived.
 
@JustTheBast  
Sounds a bit morbid if you state it like that…

Luna:  Let’s at least wait until all our current kids are dead.

@klamnei
 
Thus started the Great Alicorn Foal Arms Race

@musicman722  
Your making it sound like a Pony porno parody of Jurassic Park where Twilight and Cadance hunts them down for baby making.  
“You understand Shining Twilight doesn’t want to snuggle Twilight was to bree…Shining did you lock the door to the safe room?”  
“No I thought you had the door.”  
Pans over to a ajar door and empty hallway. Then Shining walks over to the door to close it but then is pulled out of the safe room by a Pink blur causing Luna to run over slam the door shut and as she’s locking the camera slowly brings a purple blur into focus show Twilight was hiding right behind the opened door.

@Sine Wave  
Or she just ages really well.

@musicman722
 
Enter Gleaming Shield.

@Otterhand
 
then Cadance learns of it and tries to compete with how many babies they put out. I can imagine Luna trying to run to Shining to protect her when Twi realizes she can get ahead by having both of them be pregnant.

@Sine Wave  
Yeah, I was kind of hoping RSB would leave it up in the air so people could make their own conclusions. Oh well.

couple days later one of the kids (or all three of them) get a letter from Mama Luna.  
“GET TO WORK ON GETTING US GRANDCHILDREN YOUR MOTHER LOOKING OVER HER OLD CONTRACEPTIVE SPELLS!!!”
 
Edit  
Nightmare MAMA Twilight: “THE BABIES SHALL LAST FOREVER!!!”

Ah, this confirms Twi as an immortal in this universe

You only miss them because it’s been just long enough for you to forget what it was like to actually have them.