What Are Some Words For Sibling Pairs? Published October 28, 2010 These days, giving birth to twins is increasingly common. As many as 5 out of every 1,000 babies born shared a womb with their sibling. While cute and adorable are some starter words to use when welcoming new babies, here’s a look at the lexicon to haul out when welcoming multiples! Fraternal twins vs. identical twins Fraternal twins are twins who derive from separately fertilized ova and who have different genetic makeup. They’re also sometimes called dizygotic twins because they develop from two different zygotes. Although they’re traditionally carried together in the womb at the same time, fraternal twins may be of the same or opposite sex. While some fraternal twins resemble each other, some look as different as second cousins. Naturally, identical twins do closely resemble each other and are always of the same sex. Identical twins develop from a single fertilized ovum and share the same genotype—meaning they share the same genetic stuff. The degree of shared DNA is called zygosity, yet another word that relates to the zygotes. Unlike fraternal twins, identical twins are called monozygotic twins. Conjoined twins Some identical twins are congenitally united, which means that they were born with their bodies joined together in some manner, having failed to separate completely in utero. They were once referred to as Siamese twins, a term derived from a famous pair of conjoined twins named Chang and Eng who lived from 1811–1874. The twins were born in Siam, now named Thailand, and were joined at the chest. The term Siamese twins has fallen out of use though—conjoined twins is the preferred option. Conjoined twins are so rare that statistics aren’t available to measure their incidence. Taken together, all types of twins make up about 1.9% of the world population. What if you’re not a twin? If you’re not a twin, then it’s likely that you’re a singleton. Card players are likely familiar with the term—a singleton is also a card that is the only one of a suit in a hand. It is also a phrase used to describe chronically single young people popularized by Bridget Jones’s Diary. So, there’s that. Then again, you may be a triplet, which means one of three children or offspring born at the same birth. You could also be a quadruplet, a sextuplet, a septuplet, or even an octuplet—but we’re highly doubting that’s the case.