BACKGROUND: There has been one reported case of porencephaly diagnosed postnatally as a result of amniocentesis. We report the first antenatal diagnosis of porencephaly, which apparently resulted from penetration of the fetal skull during amniocentesis. CASE: A 37-year-old woman had genetic amniocentesis at 16 weeks' gestation. The fetal skull was penetrated during the unguided procedure. Porencephaly discovered at 28 weeks' gestation was confirmed by imaging studies in the neonatal period. CONCLUSION: Inadvertent fetal skull penetration appeared to result from amniocentesis unguided by continuous ultrasound.