In 1866, Langdon Down reported that the skin of individuals with trisomy 21 appears to be too large for their body. In the 1990s, it was realized that the excess skin of individuals with Down’s syndrome can be visualized by ultrasonography as increased nuchal translucency in the first 3 months of intrauterine life. Fetal nuchal translucency thickness at the 11–14-week scan has been combined with maternal age to provide an effective method of screening for trisomy 21; for an invasive testing rate of 5%, about 75% of trisomic pregnancies can be identified. When maternal serum free-b human chorionic gonadotropin and pregnancy-associated plasma protein-A at 11–14 weeks are also taken into account, the detection rate of chromosomal defects is about 90%.

In addition to its role in the assessment of risk for trisomy 21, increased nuchal translucency thickness can also identify a high proportion of other chromosomal abnormalities and is associated with major defects of the heart and great arteries, and a wide range of skeletal dysplasias and genetic syndromes. Possible mechanisms for increased nuchal translucency include cardiac failure, venous congestion in the head and neck due to superior mediastinal compression, altered composition of the extracellular matrix, abnormal or delayed development of the lymphatic system, failure of lymphatic drainage due to impaired fetal movements, fetal anemia or congenital infection.

Other benefits of the 11–14-week scan include confirmation that the fetus is alive, accurate dating of the pregnancy, early diagnosis of major fetal defects, and the detection of multiple pregnancies. The early scan also provides reliable identification of chorionicity, which is the main determinant of outcome in multiple pregnancies.

As with the introduction of any new technology into routine clinical practice, it is essential that those undertaking the 11–14-week scan are adequately trained and their results are subjected to rigorous audit. The Fetal Medicine Foundation, under the auspices of the International Society of Ultrasound in Obstetrics and Gynecology, has introduced a process of training and certification to help to establish high standards of scanning on an international basis. The Certificate of Competence in the 11–14-week scan is awarded to those sonographers that can perform the scan to a high standard and can demonstrate a good knowledge of the diagnostic features and management of the conditions identified by this scan.

The 11-14-week scan
Copyright © 2001 by KH Nicolaides, NJ Sebire, RJM Snijders & RLS Ximenes
produced at Centrus ®