Objective To determine the value of early pregnancy sonography in detecting fetal abnormalities in an unselected obstetric population.Design Prospective cross-sectional study. All women initially underwent transabdominal sonography and when the anatomical survey was considered to be incomplete, transvaginal sonography was also performed (20.1 %). Nuchal translucency was measured and karyotyping was performed as appropriate.Setting University Department of Obstetrics and Gynaecology.Participants 6634 sequential unselected women (mean maternal age 29-9 years, range 13-50; mean gestational age 12& weeks, range 11+0-14+6), carrying 6443 live fetuses participated in this study.Main outcome measure Detection rate of fetal anomalies and the associated cost per case detected in early pregnancy. ResultsThe incidence of anomalous fetuses was 1.4% (926443) including 43 chromosomal abnormalities. The detection rate for structural abnormalities was 59.0% (37/63, 95% CI 46.5-72.4) and the specificity was 99.9% in early pregnancy. When the first and second trimester scans were combined, the detection for structural abnormalities was 8 1.0% (5 1/63, 95% CI 67.7-89.2). Seventy-eight percent (31/40) of chromosomal abnormalities (excluding three cases of XXY) were diagnosed at 11-14 weeks, either because of a nuchal translucency greater than or equal to the 99th centile for gestational age (43%; 17/40,95% CI 27-4-60.4), or due to the presence of structural abnormalities (35%; 14/40, 95% CI 21-2-52.8). Sixty-five percent (15/23) of cases of trisomy 21 were also diagnosed either because of having a nuchal translucency greater than or equal to the 99th centile (57.0%; 13/23) or due to the presence of a structural abnormality (9.0%; 2/23). Overall, the detection rate of structurally abnormal fetuses was 59% (37/63) in early pregnancy and 81% in combination with the second trimester scan. The cost per abnormality diagnosed in early pregnancy is estimated to be f6258 per structurally abnormal fetus, f7470 per chromosomal abnormality and f4453 per anomalous fetus. ConclusionThe majority of fetal structural and chromosomal abnormalities can be detected by sonographic screening at 11-14 weeks, but the second trimester scan should not be abandoned.

Whilst the accuracy of sonographic determination of fetal gender at 11-14 weeks is good, it still falls significantly short of invasive karyotyping tests.

Objective To investigate the role of first trimester sonography in detecting chromosomal abnormalities in an unselected obstetric population. Conclusions A significant proportion of fetal chromosomal abnormalities can be detected by first trimester sonographic screening to assess fetal structural appearance. The sensitivity of detection can be improved by combining measurement of nuchal translucency with detailed examination of fetal anatomy.

Although the majority of aneuploidies were detected by increased nuchal translucency and/or the presence of structural abnormalities (78%; 25/32), the use of soft ultrasonographic markers in the first trimester would have increased the overall detection by a further 3%. These data are preliminary and many thousands of pregnancies will need to be examined to determine the significance of the individual markers in different chromosomal abnormalities.

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