Midtrimester screening for open neural tube defects: correlation of sonography with amniocentesis results.

Eight pregnancies with open neural tube defects were detected in 70 midtrimester patients referred for sonography and amniocentesis because of elevated maternal serum alpha-fetoprotein levels. Two cases of anencephaly were detected by sonography, aborted without amniocentesis, and confirmed by pathologic examination. Six cases of open spina bifida were detected through amniocentesis by elevation of alpha-fetoprotein and acetylcholinesterase levels. In only three of these six was the abnormality seen on sonography. All six pregnancies were terminated, and open spina bifida defects were confirmed on pathologic examination. Amniocentesis was 100% sensitive for diagnosis of open spina bifida, while sonography was only 50% sensitive. Our results indicate that, in patients with elevated serum alpha-fetoprotein and a normal fetal sonogram, amniocentesis should be performed to rule out open spina bifida.