Incomplete separation of fetuses; such fetuses are monochromic and monoamniotic with an incidence of 1 in 52000 births.

·         Thoracopagus — joined at thorax 70% (a)

·         Omphalopagus — anterior abdominal wall (b)

·         Craniopagus — skull (c)

·         Pyopagus — buttocks and sacrum(d)

·         Ischiopagus — ischium (e)

·         Diprosopus — two faces. (f)

a-b        c

·         Separate fetuses with a constant point of connection.

·         Absent dividing membranes.

·         Umbilical arteries may have differing Doppler wave form.

·         Associated anomalies, particularly in diprosopus.