Implications of a first trimester Down syndrome screening program on timing of malformation detection

OBJECTIVE: To determine the impact which introduction of the 11-14 week scan has had on the gestational age at which fetal malformations are detected by ultrasound in an unselected population of pregnant women. DESIGN: Retrospective study. SETTING: University hospital, Copenhagen, Denmark, covering the period 1 January 2003 to 30 June 2007. POPULATION: All pregnant women who chose a nuchal translucency scan at 11-14 weeks for Down syndrome risk estimate, and a scan at 18-20 weeks to screen for fetal malformations. METHODS: Review of cases detected ante- and postnatally. MAIN OUTCOME MEASURES: Detection rates at 11-14 weeks relative to all malformations in the population. RESULTS: A total of 216 anomalies were detected in 200 fetuses among the 9 324 fetuses included, while 70 anomalies were diagnosed in 59 infants postnatally. The prevalence of fetuses with anomalies was 2.8% (259 of 9 324). After excluding cases of pyelectasis (127), which may be considered physiological and transitory changes, the prevalence of malformed fetuses was 1.4% (132 of 9 324). Of the malformations detected antenatally, 25.8% were detected before week 15 and 59.6% in weeks 16-22. The remaining 14.6% of malformations were discovered after week 22. Among the lethal malformations, 50% were diagnosed before week 15. CONCLUSIONS: Although the purpose of the first trimester scan is to screen for Down syndrome and not for malformations, the introduction of the 11-14 week scan has resulted in the detection of approximately 26% of fetal malformations.