The aim of this prospective screening study was to evaluate the implementation of an additional ultrasound examination, incorporating the measurement of fetal nuchal translucency thickness, at 10-13 weeks' gestation in two maternity units providing routine antenatal care. During the 1 year prior to the introduction of the first-trimester scan, the major indication for fetal karyotyping was maternal age > or = 35 years and only two out of the total of 11 cases of trisomy 21 were identified. In the first 5 months of the study, 70% of the women delivering in these hospitals attended for measurement of fetal nuchal translucency thickness and the measurement was obtained in all cases. This was achieved without an increase in the number of sonographers or ultrasound machines. The incidence of fetal nuchal translucency thickness > or = 2.5 mm was 3.6% (63 of 1763), and this group included three of the four fetuses with trisomy 21. The findings of this study demonstrate the feasibility of introducing scanning at 10-13 weeks' gestation and the measurement of fetal nuchal translucency thickness in routine maternity units. The sensitivity and specificity of this method of screening are at present being evaluated in a large multicenter study.