Objective To compare the success rates of obtaining optimal 20 + 2 (2 overview + 20 planes) International Society of Ultrasound in Obstetrics and Gynaecology (ISUOG) planes for foetal ultrasound structural screening between pregnant women at gestational age (GA) 18–20 weeks and 20–22 weeks 6 days Methods A prospective descriptive study was conducted. Singleton pregnant women at GA 18–22 weeks + 6 days attending antenatal clinic between December 2019 and March 2020 were invited to participate in the study. Women whose foetuses had obvious structural anomalies were excluded. The ultrasound screening using 20 + 2 ISUOG protocol was performed by 21 operators who had completed the online ISUOG basic training programme with an experience of ultrasound scanning of at least 30 cases. The success rates of achieving optimal planes between GA 18–20 weeks and 20–22 weeks 6 days were compared using Chi-square test. Common suboptimal planes in the ultrasound scan were also presented. Results Optimal 20 + 2 ISUOG planes were successfully assessed in 97/126 (77%) and 112/126 (88.9%) patients in the group with a GA < 20 weeks and in the group with a GA ≥ 20 weeks, respectively. Overall success rate was 82.9%. The success rate for the GA < 20 weeks group was significantly lower than that for the GA ≥ 20 weeks group. The group with a GA ≥ 20 weeks had a 1.2 times higher success rate than the group with a GA < 20 weeks. The most common suboptimal planes were the facial planes, especially the median facial profile, and foetal thoracic planes. Conclusions We prefer to perform foetal structural screening using US with the 20 + 2 ISUOG protocol at a GA 20 to 22 weeks and 6 days with the aim reducing the need for repeat scans.

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