Ultrasonographic examinations were performed on 1,000 pregnant women between the 28th and 40th weeks of gestation who had been seen at a private clinic. For all women the date of last menstruation was known, and none had obstetrical or clinical-surgical complications. All were singleton pregnancies, and cases of fetal malformation were excluded from the study. The proximal humeral epiphyseal ossification center was visualized significantly frequently after the 38th week (P < 0.001). With respect to a gestational age of more than 38 weeks, its specificity was elevated (99%), as also were its positive (91%) and negative (93%) predictive values. Its sensitivity, however, was low (58%). Routine determination of the proximal humeral ossification center is a valuable aid for the estimation of gestational age in the third trimester.