Interpretation
The direct relationship of chromosomal anomalies with increasing nuchal translucency measurements is well established in fetuses between 45 mm and 84 mm in several studies.8, 9 However, there is little known about this relationship in fetuses under 45 mm. A prospective cohort study of 672 fetuses concluded that a nuchal translucency measurement above the 95th percentile in fetuses at 9 to 10 weeks could be used clinically as a marker for aneuploidy.5 The authors reported that NT was above 95th percentile in 64% of the fetuses with trisomy 21, 71% of those with trisomy 13 or trisomy 18 and in all cases of monosomy X.5 Scholl and Chasen10have compared the outcomes of fetuses with cystic hygroma both with CRL < 45 mm and CRL ≥ 45 mm and concluded that the rate of chromosomal abnormalities and birth outcomes were lower in the CRL < 45 mm fetuses with cystic hygroma as compared to the ones with CRL ≥ 45 mm. The cases in the mentioned study represent the extreme end of the nuchal oedema group (median nuchal measurement 5.5 mm),10 while our cases included nuchal measurement over 2.2 mm and had a median nuchal thickness of 3.1 mm. The results of this study reinforce the value of a pre-NIPT ultrasound and suggest an increased risk of chromosomal and structural anomalies in fetuses with early subcutaneous oedema.
The high incidence of chromosomal and structural abnormalities in cases of fetal oedema in early pregnancy found in this study also suggests that in these cases and particularly in those with marked oedema or hydrops, cell-free DNA testing may not be the ideal screening modality, despite its high accuracy in detecting the most prevalent trisomies. Women facing abnormal findings before 11 weeks of gestation should receive appropriate genetic counselling and may opt for invasive testing for genetic analysis. Regardless of which investigation method is chosen, a detailed specialized anatomical assessment of the fetus at 11 to 13+6 weeks is needed, given that a significant proportion of these cases, with or without chromosomal abnormalities, will present with structural defects.
The rate of resolution of nuchal oedema at the 11 to 13+6 weeks ultrasound in our series was 81.9%. Although resolution of increased nuchal thickness before 14 weeks has been previously described in only one fifth of the fetuses with early nuchal enlargement, persistence of an increased nuchal translucency has been consistently associated with adverse outcomes.11