<unk> is a type of cephalic disorder. This literally means absence of the head. It is a much rarer condition than anencephaly. The<unk> c fetus is a parasitic twin attached to an otherwise intact fetus. The<unk> c fetus has a body but lacks a head and heart; the fetus's neck is attached to the normal twin. The blood circulation of the<unk> c fetus is provided by the heart of the twin. The<unk> c fetus can not exist independently of the fetus to which it is attached.  Similarly, acardiac fetuses lack everything from the heart upwards, including head and arms, and behave similarly to<unk> c fetuses.
Acephaly 
Microcephaly
Plagiocephaly
Macrocephaly
Brain zap
Acrania
Deformational plagiocephaly
Iniencephaly