A rare disorder where some of the embryonic tissues that develop into the lower spine, genitalia and lower abdominal organs are duplicated - probably due to the incomplete separation of twins arising from one egg. The range of possible defects is extensive but often they are able to be surgically corrected and a relatively normal life is possible. See also Caudal duplication

Dipygus, also known as "dicephalus parapagus conjoined twins," is a rare and severe congenital abnormality in which twins are born fused together from the waist down, resulting in two separate pelvises and four legs. This is one of the most rare forms of conjoined twins, and it is estimated to occur in about 1 in every 1.5 million births.

The term "dipygus" is derived from the Greek words "di," meaning two, and "pyge," meaning buttocks. In cases of dipygus, the twins usually share a common gastrointestinal, genitourinary, and reproductive system. They may also share a spinal cord and may have abnormalities in other organs, including the heart, lungs, and kidneys.

The exact cause of dipygus is unknown, but it is believed to occur as a result of an abnormality in embryonic development, specifically during the process of twinning. Dipygus is usually diagnosed prenatally using ultrasound imaging, and in rare cases, it may be diagnosed at birth.

There is no cure for dipygus, and treatment options depend on the specific medical needs of each individual case. In some cases, surgery may be necessary to separate the twins or to correct any abnormalities. Other treatment options may include physical therapy, occupational therapy, and medical management of any associated medical conditions.

The long-term outlook for individuals with dipygus depends on the severity of their condition and any associated medical conditions. In general, the prognosis for dipygus is poor, and most cases are incompatible with life. However, there have been rare cases of individuals with dipygus who have survived into adulthood and lived relatively normal lives.