In Greek mythology, a Chimera is a fire-breathing monster with the head of a lion, the body of a goat, and the tail of a snake. Scientifically speaking, a chimera is someone who has multiple sets of DNA. If every person has a unique DNA signature, then a chimera would be two people rolled into one, and this is actually kind of what’s happening. This can occur in utero when an embryo takes in its twin’s cells, and therefore genetic material. Identical twins happen when a zygote splits. The moment the zygote splits is extremely important because the later the zygote separation occurs, the more genetic material the two fetuses can share. Two fetuses that don’t entirely split can become conjoined twins. If one dies, it’s the same thing as losing a pregnancy early, while the remaining baby can be healthy.
It’s twin death in the first trimester that scientists are particularly interested in. When one twin dies, its material is sometimes absorbed into the surviving embryo causing what is known as tetragametic chimerism. This is more commonly known as Vanishing Twin Syndrome. The surviving embryo will develop with both its own and its twin’s DNA, meaning the child will effectively be born its own twin. Vanishing Twin syndrome is way more common that you might think: In 1945, Dr. Stoeckel was the first to suggest that the rate of multiple conceptions was greater than multiple birth rates. Therefore more zygotes divide than grow into fully formed twins and the dead twin material is reabsorbed by the living twin without a trace–unless the surviving twin’s DNA is studied. It’s estimated that about one in every eight single childbirths start as multiple pregnancies with cells from the miscarried sibling occasionally absorbed by the survivor.
