OBJECTIVE: To evaluate the performance of the combined test for first-trimester screening for trisomy 18 and 13, when the double test is scheduled weeks before the nuchal translucency scanning.

METHODS: The study included all 40 cases of trisomy 18 and 13 from April 2004 to October 2008 in a screening programme, where the double test was measured in gestational weeks 8 + 0 to 13 + 6 and the nuchal translucency in weeks 11 + 2 to 13 + 6.

RESULTS: Twenty-eight among the 40 cases had complete information on all variables in the first-trimester screening test. Among 19 cases having the double test taken before 10 + 0 weeks, 10 cases were detected (detection rate (DR) = 53%) and among 9 cases having the double test taken after 10 + 0 weeks, 6 cases were detected (DR 67%). There was no significant difference in the DRs (p = 0.48). A total of 29 cases were detected at the first-trimester screening, resulting in an overall DR for trisomy 18 and 13 at 73%.

CONCLUSION: This study showed no significant differences in the trisomy 18/13 DRs when grouped according to having the double test taken before or after 10 + 0 weeks. The DR was 73% at the first-trimester screening.