Nuchal Translucency Scan: Step by StepChapter 1

Nuchal translucency (NT) is a sonographic appearance of subcutaneous accumulation of fluid behind the fetal neck in the first trimester of pregnancy.

The term translucency is used, irrespective of whether it is septated or not and whether it is confined to the neck or enveloped the whole fetus.

The incidence of chromosomal and other abnormalities is related to the size, rather than the appearance of NT.

During the second trimester, the translucency usually resolves and in a few cases, it evolves into either nuchal edema or cystic hygromas with or without generalized hydrops.

The gestation should be 11–13+ 6 weeks and the fetal crown-rump length should be 45–84 mm.

A mid-sagittal section of the fetus should be obtained and the NT should be measured with the fetus in the neutral position.

Only the fetal head and upper thorax should be included in the image. The magnification should be as large as possible and always such that each slight movement of the calipers produces only a 0.1 mm change in the measurement.2

The maximum thickness of the subcutaneous translucency between the skin and the soft tissue overlying the cervical spine should be measured. Care must be taken to distinguish between fetal skin and amnion.

The callipers should be placed on the lines that define the NT thickness—the crossbar of the calliper should be such that it is hardly visible as it merges with the white line of the border and not in the nuchal fluid.

During the scan, more than one measurement must be taken and the maximum one should be recorded.

Appropriate training of sonographers and adherence to a standard technique for the measurement of NT are essential prerequisites for good clinical practice.

The success of a screening program necessitates the presence of a system for regular audit of results and continuous assessment of the quality of images.

Training is based on a theoretical course, practical instruction on how to obtain the appropriate image and make the correct measurement of NT, and presentation of a logbook of images.

Ongoing quality assurance is based on assessment of the distribution of fetal NT measurements and examination of a sample of images obtained by each sonographer involved in screening.

The risk for trisomies is derived by multiplying the a priori maternal age and gestation-related risk by the NT likelihood ratio.3

The NT likelihood ratio depends on the degree of deviation in fetal NT measurement from the normal median for that crown-rump length.

In trisomy 21 pregnancies at 11–13 + 6 weeks, the maternal serum concentration of free β-hCG is higher (about 2 MoM) and PAPP-A is lower (about 0.5 MoM) than in chromosomally normal fetuses.

There is no significant association between fetal NT and maternal serum free β-hCG or PAPP-A in either trisomy 21 or chromosomally normal pregnancies. The ultrasonographic and biochemical markers can be combined to provide more effective screening than either method individually.

Prospective studies, in more than 50,000 pregnancies, including more than 250 fetuses with trisomy 21, have demonstrated that screening by a combination of fetal NT and either first or second trimester maternal serum biochemistry can identify 85–90% of fetuses with trisomy 21 for a false-positive rate of 5%.

In trisomies 18 and 13 maternal serum free β-hCG and PAPP-A are decreased. In sex chromosomal anomalies maternal serum free β-hCG is normal and PAPP-A is low. In paternally derived triploidy maternal serum free β-hCG is greatly increased, whereas PAPP-A is mildly decreased. Maternally derived triploidy is associated with markedly decreased maternal serum free β-hCG and PAPP-A. Screening by a combination of fetal NT and maternal serum PAPP-A and free β-hCG can identify about 90% of all these chromosomal abnormalities for a screen positive rate of 1%, in addition to the 5% in screening for trisomy 21.

Increased fetal NT thickness at 11–13 + 6 weeks is a common phenotypic expression of chromosomal defects and a wide range of fetal malformations and genetic syndromes.

The prevalence of fetal abnormalities and adverse pregnancy outcome increases exponentially with NT thickness. However, the parents can be reassured that the chances of delivering a baby with no major abnormalities is more than 90% if the fetal NT is between the 95th and 99th centiles, about 70% for NT of 3.5–4.4 mm, 50% for NT 4.5–5.4 mm, 30% for NT of 5.5–6.4 mm, and 15% for NT of 6.5 mm or more.

The vast majority of fetal abnormalities associated with increased NT can be diagnosed by a series of investigations that can be completed by 14 weeks of gestation.

In monozygotic twins, the risk for chromosomal abnormalities is the same as in singleton pregnancies.

In monochorionic pregnancies, the false-positive rate of NT screening (8% per fetus or 14% per pregnancy) is higher than in dichorionic twins, because increased NT is an early manifestation of TTTS.

The risk for aneuploidy is calculated for each fetus, based on maternal age, fetal NT and maternal serum biochemistry and then the average risk between the two fetuses is considered to be the risk for the pregnancy as a whole.

Screening by a combination of fetal NT and maternal serum biochemistry identifies about 85–90% of trisomy 21 fetuses for a false-positive rate of 10% (compared to 5% in singletons).

In pregnancies discordant for chromosomal defects the main options are either selective feticide or expectant management.