Wilma's pregnancy was due to in-vitro fertilization (IVF), in which her husband's sperm was combined with her ova in a petri dish. In an unforgivable breach of proper medical procedure, however, the pipette used to transfer material had apparently been reused after a visit from a previous sperm donor. Wilma Stuart's ova were fertilized by both men, and two of the re-implanted embryos matured into healthy young boys.
Strictly speaking the boys were only half-brothers, even though they were delivered as twins. They entered the medical literature as yet another documented case of heteropaternal superfecundation, a scientific term meaning "different fathers, multiple babies." Most such cases, however, are not the result of IVF, but rather more traditional conception methods.