Uniparous is an adjective referring to an animal that only gives birth to one baby at a time, like an elephant. (And if the elephant were born between April 20 and May 20, it would be a Taurus, or — wait for it — a bull elephant 😀 !)
Human beings are usually uniparous, though they occasionally have multiple births like twins, triplets and maybe even more. You’ll most likely find a description of an animal as uniparous in zoology class, although it’s sometimes used to describe a woman who has only had one child in her lifetime.
Or U can be for Unicorn, the mythical horse-like creature with a spiraling pointed horn on their forehead, which doesn’t really scream birthday, but I have this really cute graphic that does!
(My A to Z Challenge theme is birthdays.)