acardiac

adjective

acar·​di·​ac (ˈ)ā-ˈkär-dē-ˌak 

: lacking a heart

A normal (pump) twin provides circulation for itself and for an abnormally developing acardiac (perfused) twin. The acardiac twin is not connected to the placenta but, rather, directly to the umbilical cord of the pump twin.

—P. Malladi et al., in Pediatric Surgery and Urology: Long Term Outcomes

also : marked by the presence of an acardiac fetus

Acardiac pregnancies are extraordinarily rare; nationwide they occur in one out of every 35,000 pregnancies …

—Betsy Querna, The Herald News (Passaic County, New Jersey)