Few studies have addressed the prenatal detection rate of congenital heart defects in a non-selected population at 18 weeks of gestation. Our objective was to assess the change in the prenatal detection rate of congenital heart defects in such a population that resulted from incorporating the four-chamber view at the second-trimester routine ultrasound examination. The prenatal detection rate of heart defects was prospectively compared between 4435 fetuses in Phase I who were scanned without special attention to the heart, and 7459 fetuses in Phase II who were scanned incorporating the four-chamber view. Of the 49 heart defects in Phase 1, 17 (35%) were critical and three (18%) of these were detected prenatally. Of the 90 heart defects in Phase II, 23 (26%) were critical, six (26%) of these were detected prenatally at the 18 weeks' routine scan, and three were detected in the third trimester, providing a total prenatal detection rate of 39%. A defect was classified as critical when a surgical repair was likely to be required because of gross structural complexity having a functional significance, e.g. transposition of the great arteries, hypoplastic left heart syndrome, atrioventricular septal defect, coarctation of the aorta, and large ventricular septal defect. No non-critical heart defects were detected prenatally in either of the phases. The incidences in the total population were 11 and 12/1000 in Phases I and II, respectively. Thirty-two per cent of the critical and 16% of the non-critical defects had associated abnormalities and/or abnormal karyotype.(ABSTRACT TRUNCATED AT 250 WORDS)