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What is Ischiopagi and explain in detail?
Ischiopagi comes from the Greek word ischio- meaning hip (ilium) and -pagus meaning fixed or united. It is the medical term used for conjoined twins (Class V) who are united at the pelvis. The twins are classically joined with the vertebral axis at 180°. However, the most frequent cases usually structures the ischiopagus twins with two separate spines forming a lateral angle smaller than 90°.  The conjoined twins usually have four arms; two, three or four legs; and typically one external genitalia and anus.It is mostly confused with pygopagus where the twins are joined dorsally at the buttocks facing away from each other, whereas ischiopagus twins are joined ventrally and caudally at the sacrum and coccyx. Parapagus is also similar to ischiopagus; however, parapagus twins are joined side-by-side whereas ischiopagus twins typically have spines connected at a 180° angle, facing away from one another.

Classification
Ischiopagus Dipus: This is the rarest variety with the twins sharing two legs with no lower extremities on one side.
Ischiopagus Tripus: This is the most common variety. These twins share three legs, the third leg is often two fused legs, or is non-functioning. The twins also usually share only one set of external genitalia.
Ischiopagus Tetrapus/Quadripus: This variety has the twins at a symmetrical continuous longitudinal axis with their area of union not broken anteriorly. The axes extends in a straight line but in opposite directions. The lower extremities are oriented at right angles to the axes of the thorax and the adjacent limbs near the union of the ischium belong to the opposite twin.

Embryology
During embryonic development, twins can form from the splitting of a single embryo (monozygotic) which forms identical twins or the twins can arise from separate oocytes in the same menstrual cycle (dizygotic) which forms fraternal twins. Although the latter is more frequent, monozygotic is the reason conjoined twins can develop. In monozygotic twinning for conjoined twins such as ischiopagi, the twins form by the splitting of a bi-laminar embryonic disc after the formation of the inner cell masses. Thus, making the twins occupy the same amnion which can lead to a conjoining of the twins as a result of the twins not separating properly during the twinning process. Separation occurring between the seventh and thirteenth days should result in a monochorionic, monoamniotic identical twins sharing a yolk sac. If separation of the twins occur in the later stages of development prior to the appearance of the primitive streak and axial orientation, then it can be predicted that conjoined twins will develop. The origin of exactly what goes wrong to produce ischiopagus or any conjoined twin is a result by either incomplete fission or double overlapping inducing centers on the same germ disc. Various studies suggest that mechanical disturbances such as shaking of the blastomeres, exposure of the embryo to cold or insufficient oxygen during the early process of cleavage, grafting organizer onto gastrula or half a gastrula together, or constricting the blastula or early gastrula can cause the incomplete separation of monozygotic twins. However, studies have shown that these disturbances must happen at critical times in the pregnancy for the conjoined twins to develop.

Complications
Conjoined twins are at high risk to being stillborn or dying shortly after birth. In some cases, a healthy twin and a parasitic twin are born. The parasitic twin has no hope for survival and dies and is then surgically separated from its twin. Depending upon how the twins are attached and what is shared among them, complications can arise from surgically separating the live twin from the dead twin. In Ischiopagus cases, the children share a pelvic region along with the gastrointestinal tract and genital region. Ischiopagus twins need reconstructive surgery for the genitals and gastrointestinal tract in order for normal bowel movements and reproductive possibilities in adulthood. 
For the Ischiopagus twins that both survive from birth, complications and risks also arise. Usually if both twins survive labor, one twin will be healthy and strong, while the other is malnourished and weak