Arch Hellen Med, 17(5), September-October 2000, 463-476
The 11–14 week scan
Harris Birthright Research Centre for Fetal Medicine, King’s College Hospital, Denmark Hill,
The 11–14 week scan offers the opportunity for early assessment of risk of chromosomal abnormality by measuring nuchal translucency and enables the early diagnosis of major structural defects. The nuchal translucency measurement is a sensitive and accurate method of screening for chromosomal abnormalities. The sensitivity of the test for trisomy 21 is about 80% for a 5% screen positive rate. It is likely that the sensitivity can increase to about 90% for the same screen positive rate by adding the measurement of maternal serum hCG and PAPP-A. Increased nuchal translucency is also associated with other chromosomal abnormalities and the sensitivity is 75% for trisomy 18, 72% for trisomy 13 and 87% for Turner syndrome for a screen positive rate of 5%. Fetuses with increased nuchal translucency have a higher rate of intrauterine death and are at increased risk for heart defects, other structural anomalies and genetic syndromes. Using a nuchal translucency measurement above the 95th centile as a screening test for congenital heart defects achieves a sensitivity of about 55%. The etiology of increased nuchal translucency is complex. Possible factors include heart abnormalities in combination with the unique first trimester hemodynamics, thoracic compression, impaired venous return, reduced fetal movements, abnormal lymphatic development and altered composition of collagen. A wide spectrum of structural defects has been diagnosed by ultrasound in the first trimester and some of these are associated with increased nuchal translucency. Screening studies for diagnosis of structural anomalies in the first trimester in low risk antenatal populations by ultrasound at 11–14 weeks have shown a detection rate of 50% for major abnormalities.
Key words: Chromosomal abnormalities, Nuchal translucency, First trimester.