Targeted mid-trimester ultrasound examination: how does fetal anatomic visualization depend upon the duration of the scan?

Objective: To determine the relationship between visualization of key fetal anatomic structures during mid-trimester ultrasound examination with gestational age and duration of examination.
Methods: One hundred ultrasound examinations at 16-22 weeks' gestation were reviewed to determine the times at which key fetal anatomic features were seen. Scans were terminated at 30 min or when a comprehensive anatomic survey was complete. Exclusion criteria included multiple gestation, maternal weight>77 kg, abdominal wall scarring, and suspected fetal anomalies.
Results: Visualization of cranial anatomy including lips, face, midline, ventricles, choroid plexus, and cerebellum was achieved in 98% of patients within 30 min. The corresponding figures for spine, cardiac screening (four-chamber, aortic, and pulmonary outflow views) and for abdominal anatomy (stomach, kidneys, bladder, ventral wall, and three-vessel cord) were 91%, 91%, and 99%, respectively. A complete anatomic survey including each of the above elements was obtained by 10, 15, 20, 25, and 30 min in 8%, 31%, 53%, 72% and 81% of the subjects. Rates of complete anatomic surveys within 30 min improved by gestational age interval, from 20/30 (67%) at 16-18 weeks, to 36/44 (82%) at 18-20 weeks, and 25/26 (96%) at 20-22 weeks; this rise was primarily due to improvements in visualization of the spine and heart.
Conclusions: A comprehensive anatomical survey can be completed in 10 min or less in a minority of patients. For each 5-min time increment up to 30 min, the rate of complete surveys improves. Rates of completed anatomic surveys rise with gestational age.
(Copyright (c) 2005 ISUOG.)