Fetal Bifid Sacrum Artifact: Normal Developmental Anatomy Simulating Malformation

Purpose: To determine the clinical importance and origin of a bifid configuration of the fetal sacrum seen during routine fetal sonography.

Materials And Methods: An apparent bifid malformation of the sacral spine was seen in 24 fetuses at antenatal sonography. An attempt was made to recreate this configuration prospectively in 111 consecutive second- and third-trimester fetuses. Three cadavers were also imaged to determine the origin of this configuration.

Results: Outcome information was available for 22 of the 24 fetuses; all fetuses were normal. In the prospective study, the bifid sacrum configuration was recreated in 75 of the 111 fetuses studied. The configuration could never be produced earlier than 20 weeks gestational age, was seen in some fetuses at 20-25 weeks, and could always be seen after 25 weeks. Findings from imaging studies of the three cadavers revealed that this configuration was dependent on the demonstration of the alar ossification centers of the sacral vertebra.

Conclusion: The bifid sacrum artifact is a skewed representation of normal anatomy and should not be interpreted as a true anomaly.