To assess the ability of ultrasound to detect ovaries of normal size during pregnancy A prospective study of 329 women with a normal pregnancy course was undertaken; 68 were excluded from analysis because of an enlarged, cystic ovary. Of the remainder, 60 pregnancies were examined in the first trimester and 201 in the second or third trimester. The first group underwent transvaginal (TVS) and transabdominal (TAS) scanning. The second group underwent TAS examination only. In the first-trimester group, TVS identified both ovaries in 57 patients (95%) and transabdominal ultrasound in 20 (33.3%). In the second- and third-trimester patients, TAS visualized both ovaries in 33 patients (16.4%), and neither ovary in 120 (59.7%). Both ovaries were less visible with advancing gestational age. The right ovary showed a significant change in position during pregnancy, from about 1 cm (at 15-24 weeks) to 2.5 cm (at 30-41 weeks) cranial to the iliac spine. The left ovary was found 1 cm above the iliac spine throughout pregnancy. Transvaginal sonography is adequate for the visualization of both ovaries in the first trimester of pregnancy. With advanced gestational age, the ovaries were significantly less visible by TAS. Sonographic scanning of the ovaries in second and third trimester should be concentrated mainly at the level of the iliac spine. Poor sonographic visualization of both ovaries in late gestation may mandate the use of other imaging modalities.