Maria "Masha" Ivanovna Krivoshlyapova and Daria "Dasha" Ivanovna Krivoshlyapova (, ; 3 January 1950 – 17 April 2003) were Ischiopagus tripus conjoined twins from Russia.
They were removed from their mother's custody at birth to be studied by Soviet physiologists. Their mother was told that her daughters had died soon after their birth.
The Soviet physiologist Pyotr Anokhin was studying the separate roles of the nervous system and the blood system on the body's ability to adjust to conditions such as prolonged sleep deprivation, extreme hunger, and extreme temperature change. Butler also helped write their autobiography, which was published in Germany in 2000. She wrote a novel based on their life story,
British surgeon Lewis Spitz, who specialised in operating on conjoined twins, offered to separate them in London, but Masha was against the procedure.