Nuchal Translucency

Introduction

In 1866, Down first reported an accumulation of excessive skin in individuals with trisomy 21. In the early to mid-1990s, ultrasound (US) evaluation in the first trimester revealed an accumulation of subcutaneous fluid behind the fetal neck that could explain the apparent excess skin; this finding became known as nuchal translucency (NT). An increase in NT is now recognized to be an early presenting feature of a broad range of fetal chromosomal, genetic, and structural abnormalities. As a result, first-trimester US to assess NT has evolved from an imaging evaluation used in a research setting to a screening tool that is widely available to the general obstetric population.

Major prospective studies from the United States and Europe have confirmed that US performed for NT is a powerful prenatal marker for Down syndrome, with detection rates ranging from 63% to 77% with a 5% false-positive rate. The First and Second Trimester Evaluation of Risk (FASTER) trial showed that screening with NT-based US is even more efficient when combined with maternal serum-free beta-human chorionic gonadotropin and pregnancy-associated plasma protein-A, increasing detection rates to 87% at a 5% false-positive rate.

An increased NT measurement is not uniquely associated with trisomy 21. It is also often increased in fetuses with a variety of other genetic conditions, including trisomies 13 and 18, Turner syndrome and triploidy, as well as structural birth defects such as congenital heart defects, diaphragmatic hernia, skeletal dysplasia, fetal anemia, and neuromuscular disorders. Identification of increased NT alerts the practitioner to the risk of aneuploidy and should prompt a thorough anatomic survey for structural abnormalities and a detailed fetal echocardiogram, even in the setting of a normal karyotype.

All women should be offered prenatal screening or diagnostic testing for aneuploidy, regardless of maternal age, but with the increasing number of prenatal screening options now available, deciding which is the most appropriate test is increasingly complex. Noninvasive prenatal screening using maternal cell-free DNA (cfDNA) is rapidly transforming prenatal screening, with all tests providing a high sensitivity and specificity for trisomies 21 and 18. In high-risk obstetric populations, it is recommended that this form of testing now be offered as a first-line screening option. This population includes women 35 years or older, fetuses with ultrasonographic findings indicative of an increased risk of aneuploidy, women with a history of trisomy-affected offspring, a parent carrying a balanced Robertsonian translocation with an increased risk of trisomy 13 or trisomy 21, and women with positive first-trimester or second-trimester screening test results. Conventional screening with US for NT, however, remains the most appropriate choice for first-line screening in the general obstetric population.

It is important to note that no single test is regarded by the American College of Obstetricians and Gynecologists (ACOG) as superior to other screening tests in all characteristics, and the decision on which test to perform must be based on a number of factors, including the woman's desire for information, her prior history, cost analysis, and the local availability of reliable testing and appropriate care following an abnormal result.

Disease