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

Chapter 3

MANAGEMENT OF PREGNANCIES WITH INCREASED FETAL NT

The relation between NT thickness and chromosomal defects, miscarriage or fetal death and prevalence of major fetal defects is summarized in Table 1. On the basis of these data it is possible to estimate, for each NT group, the chances of intrauterine survival and delivery of a healthy baby with no major defects. These data are useful in counselling parents of pregnancies with increased fetal NT and in planning the appropriate follow-up investigations (Figure 1).

 


Figure 1. Management of pregnancies with increased nuchal translucency thickness

 
Fetal NT below the 99th centile

In pregnancies with fetal NT below the 99th centile (3.5 mm) the decision by the parents in favour or against fetal karyotyping will depend on the patient-specific risk for chromosomal defects, which is derived from the combination of maternal age, sonographic findings and serum free β-hCG and PAPP-A at 11-13+6 weeks.

The parents can be reassured that the chances of delivering a baby with no major abnormalities is about 97% for NT below the 95th centile and 93% for NT between the 95th and 99th centiles. Furthermore, many of the major fetal abnormalities can be diagnosed or suspected at the time of the high-resolution scan at 11-13+6 weeks.

In terms of the subsequent management of the pregnancy it would be best to carry out a detailed fetal scan at 20 weeks to determine fetal growth and diagnose or exclude major abnormalities that could not be identified at the 11-13+6 weeks scan. In the 4% of fetuses with NT between the 95th and 99th centiles, special care should be taken to firstly, confirm that the nuchal fold thickness is not increased, secondly, to examine the fetal anatomy with the knowledge that the prevalence of major abnormalities is about 2.5%, rather than 1.6% in those with NT below the 95th centile, and thirdly, to examine the fetal heart. It would be preferable if specialist fetal echocardiography is carried out but the feasibility of this will primarily depend on the availability of such service.


Fetal NT above the 99th centile

A fetal NT above 3.5 mm is found in about 1% of pregnancies. The risk of major chromosomal abnormalities is very high and increases from about 20% for NT of 4.0 mm to 33% for NT of 5.0 mm, 50% for NT of 6.0 mm and 65% for NT of 6.5 mm or more. Consequently, the first line of management of such pregnancies should be the offer of fetal karyotyping by CVS. In patients with a family history of the genetic syndromes which are associated with increased NT and are amenable to prenatal diagnosis by DNA analysis (Table 3), the CVS sample can also be used for the diagnosis or exclusion of these syndromes. In addition, a detailed scan should be carried out at 11-13+6 weeks in search of the many major abnormalities that have been reported in association with increased NT (Table 3).


Resolution of increased NT

In the chromosomally normal group, a detailed scan, including fetal echocardiography, should be carried out at 14-16 weeks to determine the evolution of the NT and to diagnose or exclude many fetal defects. If this scan demonstrates resolution of the NT and absence of any major abnormalities the parents can be reassured that the prognosis is likely to be good and the chances of delivering a baby with no major abnormalities is more than 95%.

The only necessary additional investigation is a detailed scan at 20-22 weeks for the exclusion or diagnosis of both major abnormalities and the more subtle defects that are associated with the genetic syndromes listed in Table 7. If none of these is found, the parents can be counseled that the risk of delivering a baby with a serious abnormality or neurodevelopmental delay may not be higher than in the general population.


Evolution to nuchal edema

Persistence of unexplained increased NT at the 14-16 weeks scan or evolution to nuchal edema or hydrops fetalis at 20-22 weeks, raise the possibility of congenital infection or a genetic syndrome. Maternal blood should be tested for toxoplasmosis, cytomegalovirus, and parvovirus B19. Follow-up scans to define the evolution of the edema should be carried out every four weeks. Additionally, consideration should be given to DNA testing for certain genetic conditions, such as spinal muscular atrophy, even if there is no family history for these conditions.

In pregnancies with unexplained nuchal edema at the 20-22 weeks scan the parents should be counseled that there is a 10% risk of evolution to hydrops and perinatal death or a livebirth with a genetic syndrome, such as Noonan syndrome. The risk of neurodevelopmental delay is 3-5%.

 
  • Increased fetal NT thickness at 11–13+6 weeks is a common phenotypic expression of chromosomal abnormalities 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 compled by 14 weeks of gestation.
 
Previous
 
Next