Among 2.85 million births in the period from 1981 to 1992, 44,338 (1.6 per cent) infants with major congenital malformations diagnosed at birth or in the first 28 days were notified to the national monitoring system. In 1992, 4,500 infants were notified.

The data collected by State and Territory perinatal data systems and birth defects registers provided important information about the occurrence of congenital malformations, their regional variations, the characteristics of the mothers, and the survival of malformed infants.

Anencephalus and the other neural tube defects, spina bifida and encephalocele, declined in occurrence among births between 1982 and 1992. The decrease was more pronounced for anencephalus than for spina bifida or encephalocele. Induced abortions after prenatal diagnosis of these neural tube defects increased during the same period.

Increasing trends in reported rates occurred for ventricular septal defect, hypospadias, obstructive defects of the renal pelvis and ureter, gastroschisis and trisomy 18. There was a downward trend for microcephalus. Improving ascertainment was likely to be an important factor contributing to increasing trends of some of these malformations.

Many infants with major congenital malformations, especially those with multiple malformations, had an increased risk of stillbirth or neonatal death within 28 days of birth.

Neural tube defects were more common among births to younger than to older mothers. Gastroschisis was markedly more common for younger than for older mothers. Small intestinal atresia or stenosis and exomphalos were more common among births to older mothers. The well known association of increasing rates of trisomy 21 and trisomy 18 with advancing maternal age was confirmed.

Most of the 25 selected major congenital malformations were more common in twins than in singleton births. Those with a rate in twins more than twice the rate in singletons were anencephalus, hydrocephalus, coarctation of the aorta, oesophageal atresia or stenosis, small intestinal atresia or stenosis, renal agenesis and dysgenesis, cystic kidney disease, and exomphalos. Trisomy 21 (Down syndrome) and trisomy 18 occurred slightly less commonly in twins than in singleton births.

Compared with the usual sex ratio at birth of 105 males to 100 females, there were high sex ratios in excess of 120:100 for hydrocephalus, transposition of the great vessels, hypoplastic left heart, coarctation of the aorta, cleft lip, oesophageal atresia, anorectal atresia, renal agenesis, cystic kidney disease, obstructive defects of the renal pelvis and ureter, limb reduction defects, diaphragmatic hernia, exomphalos, and trisomy 21. Major malformations that occurred more commonly in females than in males were anencephalus, spina bifida, encephalocele, microcephalus, cleft palate, congenital dislocation of the hip, and trisomy 18.

Many infants with major congenital malformations were born preterm (less than 37 weeks' gestation) or had a low birthweight (less than 2SbOg).

The lack of complete data on induced abortions of malformed fetuses in some States hampers interpretation of trends of those malformations that are often diagnosed prenatally.