I just realized something after watching the OVA. The reason why the author of this chapter had Ayabe give birth to twins was because she also had a twin sister that died at birth. It's supposed to be a happy ending for Ayabe who's had feelings of guilt over her sister her entire life to have twins that gets to live healthily and happily.
My only question is, isn't it the case in the lore that only the first kid would be an uma? idk if it said anything about twins, but shouldn't only one of the kids be an umamusume? Other than that, it was sweet