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.).
Not to mention the fact that Luna and Celestia obviously generate massive quantities of magical energy (They can move planetary bodies, for cryin' out loud!). The way I figure it, they feared how such quantities of magical energy could affect a still-developing foal. So they didn't decide to find out. Instead, they played it safe, and had a different pony carry their foal (Luna's wife Dusk Spinner, or the surrogate mother Celestia hired).
