The upper half of the body of an acardiac twin is extremely poorly developed and, sometimes, not developed at all. Head, cervical spine and upper limbs are usually absent. Edema and sonolucent areas in the upper body, consistent with cystic hygroma, are common. In contrast, the lower half of the body, although malformed, is better developed. This pattern of development may be explained by the mechanism of perfusion of the acardiac twin. Blood that enters the abdomen of the fetus is deoxygenated blood that left the normal twin. Most of the oxygen available is extracted when the blood enters the acardiac twin, allowing for some development of the lower body and extremities. Once blood reaches the upper half of the body, oxygen saturation is extremely low, halting development of this area.
The acardiac twin is thus, a parasite. It requires blood pumped from the normal twin to keep developing, putting the pump fetus at risk of high output cardiac failure. The risk is directly dependent on the size of the acardiac twin: the higher the weight of the acardiac twin, the higher the risk of cardiac failure and death for the normal twin. Overall only 50% of pump twins survive.