NLM / SCIENCE SOURCE / SCIENCE PHOTO LIBRARY NLM / SCIENCE SOURCE / SCIENCE PHOTO LIBRARY
Engraving, 1686. Shows a boy with a partial twin growing from his side. A parasitic twin (also known as an asymmetrical or unequal conjoined twin) occurs when a twin embryo begins developing in utero, but the pair does not fully separate, and one embryo maintains dominant development at the expense of the other. Unlike conjoined twins, one ceases development during gestation and is vestigial to a mostly fully formed, otherwise healthy individual twin. The undeveloped twin is defined as parasitic, rather than conjoined, because it is incompletely formed or wholly dependent on the body functions of the complete fetus. The independent twin is called the autosite.
Model release not required. Property release not required.