Placentas after uneventful pregnancies were perfused under physiologic pressure in the fetal vessels, expanded to their predelivery volume, fixated with 4% formaldehyde, and compared to their ultrasound images in pregnancy. The placentas showed a side arrangement of the fetal cotyledons in contact with the basal plate. The centers of the cotyledons showed an empty space in which the spiral arteries ended. These spaces in the placenta corresponded with transsonic areas of ultrasound. The central spaces were surrounded by a relatively dense shell of villi containing the more or less fibrotic stem villi. This explained the areas of increased echo-density, as seen by ultrasound during the last trimester of pregnancy. The fetal cotyledons, composed of a central cavity and a dense villous shell, were separated by a reticular area. The veins ended at the basal plate in these intercotyledonary areas.