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

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.

Retrospective study.

University hospital, Copenhagen, Denmark, covering the period 1 January 2003 to 30 June 2007.

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.

Review of cases detected ante- and postnatally.

Detection rates at 11-14 weeks relative to all malformations in the population.

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.

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.