Ohvist [@matthewthawes](/images/2696067#comment_9787287)
Celestia used a forbidden resurrection spell to transform a lock of King Sombra's mane she had kept with her into usable DNA in order to conceive a child with, Luna flipped out at this, but Celestia managed to convince her that Celestia NEEDS to do this because she misses her lover. After being warned that the child would be destroyed if it proves a threat to either Celestia or Equestria, the Sun Princess casts the spell and is now pregnant.
||My money is on the unborn foal being Cozy Glow because THEN it would have given Cozy a compelling reason to seek revenge on Equestria if her mother had abandoned her due to the somehow evil nature within Cozy that gets manifested through those abandonment issues which would make Twilight Sparkle becoming a Princess ruling over Equestria after Cozy Glow ids imprisoned in Tartarus and later in Stone a great tragedy instead of a triumph.||