Hydrops fetalis is defined sonographically by

The term hydrops refers to excessive accumulation of serous fluid in the body, and strictly defined, hydrops fetalis is edema of the fetus. Traditionally, the diagnosis was made after delivery of a massively edematous neonate, often stillborn. With sonography, hydrops has become a prenatal diagnosis. It is defined as two or more fetal effusions or one effusion plus anasarca.(Ref: William's Obstetrics; 25th edition)