Objective: To date, there have been no studies on the optimal timing of second-trimester ultrasound screening for fetal abnormalities. The purpose of this study was to investigate whether, of three gestational ages (18, 20 and 22 weeks), any one was associated with a significant advantage in terms of identification of abnormalities or need for further ultrasound assessment.
Design: Prospective, randomized study of second-trimester unselected pregnant women, who had had an ultrasound examination with normal results at 10-14 weeks.
Subjects and methods: A total of 1206 women were randomized into three mutually exclusive groups relating to their second-trimester appointment for a screening ultrasound examination for fetal abnormalities in the second trimester of pregnancy: Group 1 at 18-18 + 6 weeks, Group 2 at 20-20 + 6 weeks and Group 3 at 22-22 + 6 weeks. The anomaly scans were carried out according to a standardized protocol. The fetuses were examined for structural and developmental abnormalities. Uterine artery Doppler measurements, including waveform recordings, were performed in all cases. The main end-points were the need for rescanning of all or part of the fetal anatomy, fetal outcome, placental localization, and incidence of notches in the uterine artery waveform.
Results: The baseline demographic characteristics and risk factors in the three groups were similar and gestational age-related fetal measurements were comparable. There were significantly higher percentages of completed scans in Group 2 (90%) and Group 3 (88%) than in Group 1 (76%; p < 0.001), but no significant difference between those scanned at 20 and at 22 weeks. This was associated with a higher incidence of non-cephalic presentation in Group 1 (46%) than in the other two groups (36%, p < 0.001). Significant differences in completing the assessment of the thorax, heart, spine and skeleton were also observed. There was no significant difference in maternal body habitus, fetal movements or the occurrence of uterine fibroids between the study groups. The incidences of low-lying placenta and of abnormal uterine artery Doppler screening were also higher at 18 weeks than at 20 and 22 weeks (p < 0.001 for both variables), with no difference seen between Groups 2 and 3. The numbers of fetal anomalies detected in the three groups were three, two and two, respectively; these did not differ significantly between the groups.
Conclusions: This study suggests that, in an unselected pregnant population, second-trimester ultrasound screening is easier to perform and less likely to require an additional scan at 20-22 weeks than at 18 weeks.