Timing of Serial Ultrasounds

While there is an agreement that women at risk for abnormalities of fetal growth should have serial ultrasound examinations in the 3rd trimester, there is a lack of consensus on how frequently these exams should be done. The American College of Obstetrics and Gynecology recommend that an ultrasound to assess fetal growth be performed between 2 to 4 weeks; however, there have been no prospective or randomized trials to determine whether the optimal interval should be closer to 2 weeks or delayed to 4 weeks. Currently, only 60% of fetuses' with abnormal growth are detected antenatally. There is equipoise of whether ultrasound every 2 versus 4 weeks improves the detection and subsequent newborn outcomes or merely leads to an increase in false positives and unnecessary interventions. It has been proposed, that if delivery occurs within 2 weeks of the ultrasound, the antenatal detection of abnormal growth may be more accurate and hence more likely to improve outcomes. Conversely, a shorter interval may result in confusion as to whether there is actual change in growth or merely variation in the measurement technique itself. Moreover, with the cost of ultrasound averaging approximately $200 per examination, the interval between ultrasounds can have major implications on public-health costs as well as the interventions that subsequently follow if an abnormality is detected. The purpose of this randomized controlled trial is to compare the frequency of the diagnosis of fetal growth abnormalities when ultrasound assessment is performed at 2 versus 4 week intervals.