Down Syndrome and Maternal Age in Hong Kong Chinese

Maternal age specific incidence of Down Syndrome in the Chinese has, up to now, been unavailable in the literature. The incidence of Down Syndrome births and the maternal age distribution for the period 1962 to 1980 were ascertained from hospital records and registers at the University teaching hospital in Hong Kong where over 99% of the patient population are of Chinese descent. Three mathematical models were studied to derive the smoothed rates for the age specific incidence of Down Syndrome. The polynomial regression exponential model, with the equation Iny = ‐ 1.2194 – .1245x + .0049x2 showed the best fit. The overall icidence of Down Syndrome was found to be 1 in 1114 live births. A lower rate was observed in the study population along the whole range of maternal age, as compared with the Caucasian counterpart. The possibility of underdiagnosis was evaluated. The percentage of pregnant women over 35 years decreased from 16.2 in the period 1962–1970 to 8.6 in the period 1971 to 1980. Of the total Down births the contribution from those over the age of 35 years decreased from 64.7% to 54.7%. An increase in the incidence of Down births however was demonstrable in mothers aged 35 to 39 delivering during the period. Based on maternal age distribution for 1979 and 1980, a reduction of Down births by 7 and 12% will be achieved assuming a one‐third compliance rate when the cut‐off point for maternal age in the amniocentesis programme is 40 and 35 years respectively.