Prenatal head position from 12-38 weeks. II. The effects of fetal orientation and placental localization.

The possible influence of placental localization and fetal orientation on a predominant fetal head position was studied longitudinally in 10 uncomplicated pregnancies from 12 to 38 weeks by means of real-time ultrasound. Throughout gestation, when the fetal vertebral column was oriented to the left side of the mother, head positions to the right and in midline were seen most frequently, whereas a vertebral column to the right was more often associated with a head left position and a head position in midline. These findings support the suggestion that when the fetal vertebral column is in a lateral orientation relative to the mother, one side of the fetal head is more likely to be restricted by the pelvis and backbone of the mother. However, the orientation of the fetal vertebral column does not seem to be a determining factor as the incidence of lateralized head positions increased with age, while the incidence of a vertebral column to the right or left side of the mother remained unchanged. No associations were found between the localization of the placenta and either head position or fetal lie.