EP18.05: First trimester referral ultrasound following the introduction of the guidelines of the SIEOG: focus on false positives and false negatives

To analyse false positive and false negative diagnoses of first trimester referral US compared to second trimester referral US. Secondary analysis of a multicentre retrospective study of prospectively collected data including singleton pregnancies at risk for structural anomaly following 1st trimester screening US. The definition of “risk for structural anomaly” was based on either NT≥3.5mm or fetal structural anomalies suspected at 1st trimester US. Only cases undergoing referral 1st trimester US with a standardised protocol at one participating centres were included. The reference to evaluate the performance of 1st trimester US was represented by 2nd trimester referral US. 137 cases had paired referral US at 11+0-13+6 and at 19-21 weeks and 206 at 11+0-13+6 and 14-16 weeks. False negatives of 1st trimester US compared to 19-21 weeks' and 14-16 weeks' scan were 12 and 10, respectively. The former included minor cardiac anomalies (VSD, persistent LSVC), 1 case each of AVSD, ductus venous agenesis and fibular hemimelia and 4 cases of evolving anomalies (hydronephrosis, ACC, hydrops). The latter included 2 cases of major cardiac anomaly, 1 case of micrognathia and 1 case of fetal tail. False positives of 1st trimester referral US were recorded in 7 and 6 cases, respectively. These included 1 case of tetralogy of Fallot, 1 case of Blake's pouch cyst and 1 case of cleft lip. In the remaining 4 false positive cases of 1st trimester referral US evolving anomalies were recorded. Our data suggest a low rate of false positive and false negative diagnoses of 11+0-13+6 weeks' scan, the latter being most represented either by minor structural anomalies with little or no impact in terms of pre- and perinatal management and counselling or by evolving anomalies that cannot be diagnosed in the first trimester. Most of the false positive diagnoses are consistent with the natural history of the anomaly, even though over-diagnosis may be accounted as a limitation of first trimester referral ultrasound.