OP28.01: The human fetal tail: first trimester finding leading to the detection of familial pilonidal sinus

Abstract

A healthy 35 year old G2 P1 presented for first trimester screen at 12 postmenstrual weeks. A normal NT (1.4 mm), IT, nasal bone and ductal flow were observed. A 5 mm echogenic protrusion resembling a fetal tail was detected arising from the median fetal caudal portion. A detailed anatomical evaluation using high frequency transvaginal ultrasound revealed normal spine, brain and male gender. The patient was counseled regarding the differential diagnosis. Based on our evaluation this was thought to represent a transitory sign for pilonidal sinus with overall low suspicion for fetal malformation. Upon questioning, the patient reported that her first child was noted to have a deep sacral dimple after birth which was determined to be ‘normal’ by post natal sonography. Inspection of the maternal sacral area indentified a deep pilonidal sinus dimple. Of interest, the mother later reported that her sibling admitted having similar deep sacral dimple. A prominent ‘fetal tail’ was seen at 14 weeks which later disappeared by 18 post menstrual weeks. Serum screening for neural tube defect was indicative of low risk and amniocentesis, performed secondary to the presence of several sonographic markers (thick nuchal fold, short femur and choroid plexus cyst), confirmed a normal male karyotype. Following an otherwise uncomplicated pregnancy, the patient delivered a healthy full term male infant. Post natal physical evaluation revealed a deep dimple at the superior end of the buttock crease similar in appearance to the other family members and consistent with the diagnosis of pilonidal sinus. Ultrasound of the newborn's spine detected no structural abnormality. Sonographic and post natal images will be shown to highlight the natural history of the human fetal tail as a transitory sign of pilonidal sinus supporting its congenital nature. Moreover, the familial pattern we present raises the suspicion for an autosomal dominant inheritance associated with this condition.