The point of Alaric's idea is all the physical change she would have to go through to complete the pregnancy, discomfort and nausea and belly bulge, a vampire can't have children because their body is paralyzed they can't grow or age or put on weight or lose weight as a living organism can normally change, vampires or hybrids (the species not the season 7 group) do not it cannot undergo any of these physical changes in a biological way. Caroline was an exception to saving the twins as a surrogate mother, if that happens, she will also have to have an explanation as to why nature would allow this situation, as immortals and hybrid vampires cannot prolong the species that way as a human normally does.