Nah she would die from pregnancy complications

@FanOfMostEverything  
Damn straight.

@The Twelfth Doctor Whoof  
Because it’s fun to overthink this kind of thing?

@Background Pony #0D9C  

@Sixth  
Not necessarily. Spike’s egg - which we can assume is of a normal size - wasn’t that big. Horses (& ponies), like all ungulates, come out fully-formed. In the real world they can walk within hours of birth, though that’s obviously not the case in the Ponyverse.
 
There doesn’t seem to be much size difference between newborn dragons and newborn foals. Rarity could therefore probably carry an egg just fine and WHY THE HELL AM I EVEN SPECULATING ABOUT THIS?!?

No, it would come out of her vagina as a miscarriage and she would make Spike eat it.

@Fortune  
My thoughts exactly (admittedly, I made this pic after seeing an explicit image of an OC pony laying eggs and wondered exactly how the hell that could work)…course Star Wars seemed to figure it out: http://starwars.wikia.com/wiki/Reptomammal

No. Her belly would explode as the dragon couldn’t fit inside her womb, leaving a horror show in the hospital.

@EHAN  
No, the tags added that automatically…

Does shipping HAVE to involve having offspring?

Probably not, as it would be unlikely that a mammal and a reptile could successfully conceive, even if both are magic.
 
For it to happen, one of them would have to be transformed, probably Spike, just so Rarity could successfully carry to term. And then the offspring would probably be a pony, albeit one that may specialize in fire magic.

I don’t know about having an egg. But I bet if Spike knocked her up, Rarity would have a cow.

Like a platypus!

I don’t think so. My first impulse is to say that when half-dragons are involved, oviposition depends on the reproductive anatomy of the mother. If it were Barb and Elusive, then eggs would be involved.

Apparently.
 

No, Sweetie.
 
Well, maybe.
 
That’s how little we know about dragons.