To construct a normal range for the nuchal translucency measurement in chromosomally and phenotypically normal fetuses between 9 and 14 weeks' gestation. The nuchal translucency was measured prospectively in 771 chromosomally normal fetuses of women attending our antenatal clinic or prenatal diagnosis center. The nuchal translucency measurement was expressed as the median and fifth, 25th, 75th, and 95th percentiles according to completed weeks of gestation based on ultrasound measurements. The median nuchal translucency measurement increased from 0.7 mm at 10 weeks' gestation to 1.5 mm at 13 weeks. A nuchal translucency measurement greater than 2.5 mm was found in 4.6% of the fetuses at 10 weeks' gestation; the incidence increased to 8.7% at 14 weeks. In normal fetuses, there is a physiologic variation in the nuchal translucency measurement between 9 and 14 weeks' gestation. The calculation of risk for trisomies based on this measurement should take this variation into account. The adoption of a gestational age-dependent cutoff point, based on the deviation of a given measurement from the median, may reduce the number of false-positive test results requiring invasive procedures for karyotyping.