Objective: To assess the natural evolution of nuchal thickness in trisomy 18 fetuses.
Methods: Serial measurements of nuchal thickness were performed over a 1- to 5-week period in 17 fetuses with trisomy 18, from the 10th to the 20th week of pregnancy. To avoid a confounding effect due to gestational age, nuchal thickness was also expressed in standard deviations (SD) for the corresponding gestational week. In addition, the changes were assessed in terms of the presence of clinically positive thickening, considered as such when its measurement was above 2.5 SD for the corresponding gestational week.
Results: On the initial nuchal thickness measurement, sensitivity for trisomy 18 decreased from 66% on assessment at 10-13 weeks of gestation to 38% at 14-16 weeks. In serial determinations, a mean increase of 1.3 mm [95% confidence interval (CI), 0.1-2.5] was observed for a mean period of 21 days. When corrected for gestational age, the mean increase of 0.2 SD (95% CI, -1.2 to -1.6) was found to be non-significant. No clinically relevant changes were recorded on re-examination, with nuchal thickening remaining stable in 76% of cases.
Conclusion: Nuchal thickening at re-examination was observed in a similar proportion of trisomy 18 fetuses as when initially observed.