A theory about the Curious Genealogy-verse. · 5:20am Jun 26th, 2014
The reason Luna and Celestia didn't carry the foals themselves is because they were afraid of what might happen.
Stress can easily affect the pregnancies of various Terran lifeforms, and could affect Equestrian lifeforms in a similar way, and these two are in charge of ruling a large country (My guess is roughly the size of Russia, and that's at the smallest.).