Objective: To examine the longitudinal course of nuchal translucency thickness by weekly measurements between 10 and 15 weeks' gestation in normal fetuses. Methods: Nuchal translucency was measured weekly from 10 to 15 weeks' gestation in 64 fetuses with normal pregnancy outcome. The median and the fifth, 25th, 75th, and 95th percentlies were calculated. Results: Nuchal translucency measurements varied considerably with gestational age; this variation followed a fetus-specific pattern. In 94% of cases, we observed an increase followed by a steady decrease in nuchal translucency measurement. A visible nuchal translucency was found after 76 and 86 days' gestation in 97% (95% confidence interval [CI] 89, 100) and 100% (95% CI 94, 100) of the fetuses, respectively. The median nuchal translucency increased from 0.7 mm at 70 days' gestation to 1.7 mm at 91 days' gestation, after which it declined to 1.0 mm at 105 days' gestation. Conclusion: A progressive increase and subsequent decrease in nuchal translucency thickness occurs with advancing gestation in most fetuses, but the timing of the peak thickening appears to be fetus-specific. In this study, each fetus developed a visible nuchal translucency. If the nuchal translucency measurement is 0 mm before 12 weeks, it may be advisable to repeat the measurement at 12 weeks' gestation. In contrast, a nuchal translucency that cannot be measured from 12 weeks' gestation onward suggests that this temporary anatomic entity is already in its waning phase.