(1)
Department of Family Medicine, University of California, Riverside, Riverside, CA, USA
Key Points
1.
Normal delivery is marked by a characteristic fetal presentation and a stereotyped series of fetal repositions.
2.
Failure to present in the usual occiput anterior position may lead to prolongation and complications of labor and may be incompatible with vaginal delivery.
3.
Careful assessment of fetal presentation is critical to the diagnosis and management of abnormal presentations.
Background
Although fetal position during the prenatal period is variable and subject to change (especially prior to 36 weeks’ gestation), most infants will arrive head first, neck flexed with the occiput (either right or left occiput) in an anterior position. Variations from this position and presentation do occur, however, and providers should be aware of possible variants. Assessment of fetal presentation should be a routine component of late prenatal care and with all patients at the time of labor. Complications with the progress of labor, as noted earlier, should prompt reevaluation of fetal presentation and position.
Occiput Positions
Occipital position is described in relation to the anterior surface of the mother. In the usual dorsal lithotomy position, this will place the anterior surface upward. In the normal presentation, the fetus will present vertex first with the occipital portion of the skull in an anterior location (occiput anterior; occiput toward the symphysis pubis, upward in the usual dorsal lithotomy position). The occiput may, however, be either posterior (occiput away from the symphysis or downward in the dorsal lithotomy position) or transverse (occiput horizontal or to the side in the dorsal lithotomy position). With the onset of labor, most infants will already be in the occiput anterior position. Approximately 20 %, however, will be positioned in an alternative position at the beginning of labor. For most of these infants, occiput posterior or occiput transverse is only a temporary position that will revert to occiput anterior during the course of delivery. Fewer than 5 % of infants will present with occiput posterior or occiput transverse at the time of delivery.
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