Epidemiology of trisomy 21: population, peri- and antenatal data.

In Down's syndrome, incidence of 1 0/00-2 0/00 have been reported in chromosomal surveys of consecutive liveborn infants and in population studies. Much attention has been focused on the influence of the decline in mean maternal age on the incidence of Down's syndrome. Decline in incidence and unchanged incidences have been reported. For mothers over 35 years old, a rise in incidence has been found in recent years in some societies. Environmental factors or seasonal fluctation might cause this trend. Data from antenatal diagnosis show a 30% higher incidence of Down's syndrome for age groups over 35 than population studies do. Late abortions of trisomic fetuses, a high perinatal mortality, and a small rise in incidence for higher maternal ages in recent years may account for this fact. With the growing tendency toward younger maternal age at childbirth, paternal factors also have to be considered. Between 10% and 30% paternal failures have been found by nondisjunction studies applying chromosomal variants. The rate of paternal failures may reflect environmental influences and young maternal age distribution. Recently, an increase in trisomy 21 children of mothers in the age group 30-39 who are pill users has been observed. The positive sex ratio of male Down's syndrome patients has been reversed in patients born to mothers using hormonal contraception. The mortality rate, in Down's syndrome is still high in early childhood, especially perinatally and in the 1st year of life. For the late 1940s in Denmark, newborns with Down's syndrome had a mortality rate of 53% in the 1st year of life. In the late 1960s and early 1970s, the rate had fallen to 22%. The main causes of death were congenital heart disease in connection with infections, especially pneumonia. Cot death might be more common in Down's syndrome. After the age of 1 year, mortality is reduced considerably and more and more cases will survive early childhood and live to rather old ages.