It's a widely-accepted rule that vampires can't get pregnant - or at least it was, until Caroline Forbes got knocked up with Alaric and Jo's twins in The Vampire Diaries. Immortality comes at a steep price, because when the body freezes in time, the circle of life also stops dead in its tracks. Biologically, when it comes to vampires procreating, one plus one does not equal two, but The Vampire Diaries' fans know there's always a loophole. So when Jo died, the Gemini coven found a way to secure their future.
After Kai escaped from the 1903 Prison World, he decided to take revenge and wipe out his entire coven at Jo and Alaric's wedding, during season 6's penultimate episode, "I'll Wed You in the Golden Summertime." Jo bled to death in Alaric's arms, and presumably the babies she was carrying died with her. Alaric's luck changed for the better during season 7 when former Gemini-turned-Heretic Valerie uncovered a big secret. In their final moments, the members of the Gemini coven performed one last spell. Kai thought it was an attempt to banish him, but they actually saved the babies by putting them into a surrogate.
While Caroline couldn't conceive a child, she was able to carry one - in this case two - once they were in the womb. During a three year flash-forward on season 7's "Best Served Cold," viewers learned Caroline and Alaric were living together in Dallas, raising twin girls. It was a huge twist, and fans found out later in the episode how Alaric's daughters survived the wedding massacre that killed Jo. Valerie explained twins were the lifeblood of the coven, and in the past, if a pregnant mother was in danger, transferring the babies was their "magical fail-safe."
Valerie used a locator spell to find them, and the trail ended with Caroline, whose reaction echoed that of viewers. "I can't have Alaric's babies inside of me. Not only is it impossible, but it literally defies the physics of the universe." At least the series acknowledged the ridiculousness of the story arc. Alaric told Caroline that since she breathed and had a heartbeat, it wasn't outside the realm of possibility that her body could support a baby. A negative pregnancy test seemed to confirm Caroline was right until Valerie realized the coven would have cloaked them. She undid the spell, and the babies appeared during an ultrasound. Magic made the pregnancy possible, and since the babies were siphoners, they thrived in Caroline's womb, feeding off her mystical energy.
Candice King announced she was pregnant before season 7, and the show's executive producer Julie Plec teased that Caroline and Stefan would be thrown a "curveball." Some fans rolled with the plot twist, while others weren't thrilled about Caroline's unwanted and unplanned pregnancy. With Nina Dobrev's exit, King needed to pick up some of the slack, so it was either use her pregnancy to the show's advantage, or waste time figuring out how to hide her condition behind multiple objects. King's pregnancy was well-timed; showrunner Caroline Dries told TV Insider, "...we wanted to give Alaric something to hope for—a light at the end of the tunnel—after everything he'd been through last season."