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

Chapter 3

OUTCOME OF FETUSES WITH INCREASED NUCHAL TRANSLUCENCY

The relation between NT thickness and the prevalence of chromosomal defects, miscarriage or fetal death and major fetal abnormalities is summarized in Table 1 (Souka et al 2004). 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.

Table 1. Relation between nuchal translucency thickness and prevalence of chromosomal defects, miscarriage or fetal death and major fetal abnormalities. In the last column is the estimated prevalence of delivery of a healthy baby with no major abnormalities.

Nuchal translucency	Chromosomal Defects	Fetal death	Major fetal abnormalities	Alive and well
<95th centile	0.20%	1.30%	1.60%	97%
95th-99th centiles	3.70%	1.30%	2.50%	93%
3.5-4.4 mm	21.10%	2.70%	10.00%	70%
4.5-5.4 mm	33.30%	3.40%	18.50%	50%
5.5-6.4 mm	50.50%	10.10%	24.20%	30%
>6.5 mm	64.50%	19.00%	46.20%	15%

Chromosomal defects

The prevalence of chromosomal defects increases exponentially with NT thickness (Table 1; Snijders et al 1998). In the chromosomally abnormal group, about 50% had trisomy 21, 25% had trisomy 18 or 13, 10% had Turner syndrome, 5% had Triploidy and 10% had other chromosomal defects.

Fetal death

In chromosomally normal fetuses, the prevalence of fetal death increases exponentially with NT thickness from 1.3% in those with NT between the 95th and 99th centiles to about 20% for NT of 6.5 mm or more (Table 1; Souka et al 2001, Michailidis and Economides 2001). The majority of fetuses that die do so by 20 weeks and they usually show progression from increased NT to severe hydrops.

Fetal abnormalities

Major fetal abnormalities are defined as those requiring medical and / or surgical treatment or conditions associated with mental handicap. The prevalence of major fetal abnormalities in chromosomally normal fetuses increases with NT thickness, from 1.6%, in those with NT below the 95th centile, to 2.5% for NT between the 95th and 99th centiles and exponentially thereafter to about 45% for NT of 6.5 mm or more (Table 1; Souka et al 2001, Michailidis and Economides 2001).

Developmental delay

Studies on the long term follow up of chromosomally and anatomically normal fetuses with increased NT reported that the prevalence of developmental delay is 2-4% (Souka et al 2004). However, it is difficult to assess the true significance of these findings because only one of the studies had a control group for comparison. Brady et al (1998), performed a clinical follow up study of 89 children that in fetal life had NT of 3.5 mm or more and 302 children whose fetal NT was less than 3.5 mm. Delay in achievement of developmental milestones was observed in one of the children in each group.