Prenatal diagnosis of anterior sacral meningocele.

Anterior sacral meningocele is an extremely rare condition and there has been only one previous report of a prenatal diagnosis. We report the case of a 36-year-old primigravida who was referred following detection of a huge fetal pelvic cyst on routine ultrasound examination at 19 + 4 weeks' gestation. Neither fetal ultrasound nor magnetic resonance imaging (MRI) at 20 + 5 weeks' gestation could detect communication between the cyst and the spinal cord. Because extension of the pear-shaped cyst through the pelvic diaphragm down to the perineum was reminiscent of dilated vagina and uterine cervix, a tentative diagnosis of hydrometrocolpos secondary to imperforate hymen was considered. On follow-up MRI at 33 + 5 weeks' gestation, a narrow stalk connecting the pelvic cyst and the spinal canal through the anterior sacral foramen was clearly delineated, allowing us to reach the prenatal diagnosis of anterior sacral meningocele.