A six year study of the antenatal detection of fetal abnormality in six Scottish health boards.

OBJECTIVE: To assess the sensitivity of prenatal diagnosis by ultrasound and biochemical methods, to evaluate the reasons for non-detection and to make appropriate recommendations.
DESIGN: Six year observational study, during which biochemical screening for trisomy 21 was introduced and there was an increase in routine ultrasound scanning at 18-22 weeks.
SETTING: Six health boards in Scotland. POPULATION: 264,481 pregnancies, of which 862 were terminated because of fetal abnormality, and 2123 delivered with prenatally detectable major fetal abnormalities. MAIN OUTCOME MEASURES: The prenatal detection of trisomies 13, 18 and 21, and 12 major structural abnormalities, which the average ultrasonographer with average skills using average equipment would be expected to detect.
RESULTS: Serum biochemical screening improved detection of trisomy 21 from 33% to 57%. The detection rate for the major abnormalities was 62% (815/1320) and 73% (598/818) when the trisomies were excluded. 18-22 weeks scanning yielded a 92% detection rate. Of the 505 undetected cases, 15% declined prenatal screening, 46% were unscreened because they were ineligible for testing, unbooked, booked too late or scanned too early for a diagnosis to be made, 2% had findings suspicious of a chromosomal abnormality but testing was not undertaken and 37% had a negative scan at a gestation when the abnormality was potentially detectable.
CONCLUSIONS: A policy of first trimester scanning followed by serum alpha-fetoprotein screening and additional scanning as clinically indicated is effective in detecting major structural abnormalities, but scanning at 18-22 weeks and serum biochemical screening for trisomy 21 improved the detection rates. Supervised training and adequate equipment are essential. Present prenatal diagnostic tests will not detect all abnormalities and patients must be made aware of this.