The 11-14 weeks scan - KH Nicolaides, NJ Sebire, RJM Snijders, AP Souka

Introduction

In 1866 Langdon Down noted that common characteristics of patients with trisomy 21 are skin deficient in elasticity, giving the appearance of being too large for the body, and flat face with a small nose. 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 third month of intrauterine life. Fetal nuchal translucency thickness at the 11–13+6 weeks 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 ß-human chorionic gonadotropin and pregnancy-associated plasma protein-A at 11–13+6 weeks are also taken into account, the detection rate of chromosomal defects is about 85-90%. In 2001, it was found that in about 70% of fetuses with trisomy 21 the nasal bone is not visible at the 11–13+6 weeks scan and preliminary results suggest that this finding can increase the detection rate of the first trimester scan and serum biochemistry to more than 95%.

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 11–13+6 weeks 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 11–13+6 weeks scan are adequately trained and their results are subjected to rigorous audit. The Fetal Medicine Foundation, 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 weeks 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.