Prenatal ultrasonographic diagnosis of spina bifida: timing of examination.

A detailed ultrasonographic examination of the fetal vertebral column may be very close to normal at 16 weeks' gestation, yet may conclusively demonstrate spina bifida four and one-half weeks later, as exemplified in a patient we report. We suggest that if there is any reason to investigate a possible neural tube defect, a detailed ultrasonographic examination of the entire vertebral column should be made at approximately 20 weeks if spina bifida has not been demonstrated earlier.