Since talking about kids has wound up occupying the entire month, let’s stay with that — sort of — and look at what happens when you get two (or more) at once.
We probably all know that there are two basic kinds of twins, identical and fraternal. Or, to phrase it more scientifically, monozygotic and dizygotic, which refers to whether one egg split in two after fertilization or two eggs were fertilized at the same time. There are actually more options than that, but — with one exception, which I’ll mention below — they’re probably not relevant to most stories. I should, however, note that twins of different sexes are never monozygotic/identical, however closely they may resemble one another (a favorite trope to play with in narrative).
Sticking with the pragmatics a bit longer: if childbirth is hard on the body, multiple births are even harder. It’s very common for twins, let alone triplets or quadruplets or more, to be born prematurely, and for there to be complications in labor that can threaten the life of the infants, the mother, or both. Without modern or science fictional medicine, or some form of magic to occupy the same niche, you get a lot of stories and historical incidents about twins who grow up knowing they “killed their mother” — putting scare quotes around that because of course they aren’t at fault, but that doesn’t mean the rhetoric won’t go that way. The odds of successfully giving birth to triplets or more are extremely low; the good news is that multifetus pregnancies like that are also extremely rare without the intervention of modern fertility treatments.
(Assuming human biology, of course. But “what happens when alien biology comes into play” is one of the essays I intended for this month, which has been punted to some future point.)
It can actually get far creepier than that, though we didn’t know about this until relatively recently: “vanishing twin syndrome” is what happens when one fetus of a multifetal pregnancy dies in utero and its tissue absorbed by the other. Yes, twins can compete for maternal resources even before they’re born! It’s apparently far more common than you’d believe, and may be one end of a spectrum of biological processes that can also produce conjoined twins.
Those latter are quite rare, and depending on where the conjunction is, their life expectancy can be absolutely dreadful: half are stillborn, and of those who are born alive, one-third die within a day. Attempts to surgically separate them go back as far as the tenth century (though in that case one twin had already died, and the surgeon was hoping to save the other), but it’s not until the twentieth century that we began to do it at all well. When the conjunction involves vital organs, though, this can be ethically problematic, as sometimes it means accepting the death of one twin in order to give the other a better long-term medical outcome.
But let’s look at the kinds of twins that tend to show up in stories, which are not so grim a medical scenario.
There is a strong tendency for societies to assign some kind of special value to twins, be it positive or negative. In modern times we often imagine that twins have a special connection — and we’re not entirely wrong! There’s a high incidence of young twins developing their own private language they use to communicate with each other (the technical term for this is cryptophasia), though they often lose this as they get older. In general, the fact that the pair grow up together and are often treated as a unit by the adults around them means they’ll have a very close bond.
But beliefs about twins go well beyond that ordinary level. Some cultures believe they don’t just share a bond; they literally share one soul in two bodies. As a result, they may have magical powers — but they may also be vulnerable to misfortune if one of the pair dies. In Yoruba tradition, the ideal response to this is to carve an Ibeji doll to represent the deceased twin, which is cared for as if it were a living person.
Those magic powers aren’t necessarily a good thing, though. Even within the confines of Nigeria, attitudes can vary widely: the Efik — neighbors of the Yoruba — traditionally view twins as a bad omen, evil spirits masquerading as human. As such, they might be targets of the infanticide we talked about before. In other cases, twins are dualistic: one is good and one evil, leaning into our narrative love for things in opposition. Extended outward, this can echo the European concepts of the doppelgänger, fetch, and other terms for someone’s supernatural double, which is almost invariably an ill omen.
Or sometimes the negative beliefs are more mundane. While it’s technically possible for a woman to become pregnant twice in one cycle by two different men — a phenomenon known as heteropaternal superfecundation — it’s extremely rare among humans, making up (so Wikipedia tells me) a mere 2.4% of dizygotic twins whose parents got into paternity lawsuits. In the general population of twins, that percentage will likely be even smaller. But before we had the medical knowledge to know how fertilization works, quite a lot of Europeans believed that twins of any kind were proof of infidelity: how else could a woman possibly give birth to two children at once? Twins might therefore be deemed automatic bastards, when in fact they’re simply accidents of biology.
Like foundlings and orphans, we love to make use of twins in our stories. We particularly love tales where they were separated at birth, encountering each other many years later and eventually realizing they’re related — cue the hugs! These tales often closely resemble those around foundlings, with the twins having some kind of mark or token that allows them to recognize their kinship. Or if the pair are identical, hijinks ensue as one is mistaken for the other, before they finally show up in the same place at the same time and the confusion is at last resolved. (Bonus points if one or both of the twins is cross-dressing.)
If, on the other hand, the genre in play is horror, the narrative may go in a very different direction. Here you might have a kid haunted by the ghost of a twin who died in utero or shortly after birth, who either needs to be laid to rest (the happy version) or exorcised like the malevolent threat it’s become (the not-so-happy version). You can also imagine a variety of fantastical possibilities, like the lost twin being a guiding spirit or the conduit that allows the surviving one to become a great worker of magic.
How actual twins feel about this will vary. Based on the ones I’ve encountered, some enjoy the idea that their particular situation gives them special powers or status, while others roll their eyes and wish they could just be treated like anybody else. Which is to say that, just like ordinary people, they don’t all agree — sometimes not even with their own twin.