Ultrasonic evaluation of fetal kidneys.

Fetal renal morphology, technique and frequency of reliable visualization of the kidneys, and renal size were evaluated in 177 apparently normal fetuses. The kidneys were not reliably identified before 15 post-menstrual weeks and were seen in fewer than 50% of cases prior to 17 weeks. However, between 17 and 22 weeks (the critical time for genetic counseling) one or both kidneys were seen in 90% of cases, with both being identified in 75%. After 20 weeks, one or both kidneys were identified in approximately 95%. With high-resolution equipment, positive identification of one or both fetal kidneys should be possible in a large percentage of cases after 20 post-menstrual weeks.