Human fetal sacrococcygeal extension or 'tail' in the second trimester: prenatal diagnosis, associated findings, and clinical outcome.

Background: There are over 30 cases of prenatally diagnosed sacral extensions or human 'tails' in the literature, including isolated and syndromic etiologies. Most cases were reported to resolve by the second trimester and postnatal course was benign. Our objective was to describe the prenatal findings, associated anomalies, and clinical outcome of a series of seven fetuses diagnosed prenatally with fetal sacrococcygeal extension.

Methods: In a prospective study on a series of 4040 consecutive patients seen through our clinic in a 10-year period, 7 patients had a sacrococcygeal extension identified on fetal imaging. Fetal sonographic and magnetic resonance imaging findings, results of genetic testing, and clinical outcomes were analyzed.

Results: All seven fetuses had associated anomalies, including neurologic, craniofacial, cardiac, spinal, renal, and musculoskeletal abnormalities, or abnormal ultrasound findings including growth restriction and abnormal umbilical cord Doppler. There were two cases of trisomy 13 and one case of Pfeiffer syndrome. The mean gestational age at diagnosis was 19.3 weeks.

Conclusion: When a sacrococcygeal extension is seen on fetal imaging, work-up should include detailed fetal imaging and karyotype, and fetal growth should be monitored. Our experience with fetal sacrococcygeal extension demonstrates variability in the underlying presentation and etiology, making counseling in a prenatal setting challenging.