Fetal and early neonatal atrial measurements correlating with plasma atrial natriuretic peptide were studied to evaluate the circulatory change before and after delivery in a longitudinal study of 10 normal fetuses from 1 week before until 5 days after delivery. Before delivery, right atrial area value was significantly larger than that of left atrial area; however, there were no significant differences between left and right atrial areas after delivery. Right atrial area value before delivery was significantly larger than those after delivery, whereas the left atrial area did not change before and after delivery. Combined atrial area value before delivery was also larger than those after delivery. Plasma atrial natriuretic peptide values did not change after delivery. These results suggest that the change from fetal to neonatal circulation mainly caused by the pulmonary circulation induces the change of right atrial size before and after delivery.