We report simple methods for assessing the fetal heart size by calculating the ratio of the area of the four chambers to the chest area when viewed in the same plane. The average value for this ratio in normal 99 fetal hearts was 0.30 +/- 0.05 and in 16 cases of fetal cardiomegaly it was 0.42 +/- 0.05. The difference was statistically significant (P less than 0.01). A second method is presented which involves measurements of the AP diameter of the heart in the longitudinal plane of the fetal trunk divided by the AP diameter of the chest just above the liver. The average value of this ratio in the normal group was 0.49 +/- 0.06. In a group with cardiomegaly, the average value was 0.62 +/- 0.04 and the difference was statistically significant (P less than 0.01). These methods to calculate heart size use only screen data, and are a simple tool to evaluate fetal heart cardiomegaly.