A partially imaged normal fetus lacking an upper body is described as which type of twin?

• A. acardia twin
• B. pump twin
• C. normal twin
• D. recipient twin

Answer: (A)

In twin pregnancies with a shared placenta, one fetus can develop without a functioning heart in a situation known as TRAP sequence. The fetus that has a heart and normal anatomy (the pump twin) ends up supplying blood to the other twin, which receives circulation without being able to pump it itself. This results in an acardiac twin: a fetus that is partially formed, often appearing normal in some views but lacking a heart and upper body structures.

So, when ultrasound shows a twin that appears at times to resemble a normal fetus but has no heart and an underdeveloped upper body, this is characteristic of an acardiac twin. The pump twin remains the viable, functioning twin, though its heart work is increased due to supporting both fetuses.