Prenatal genetic diagnosis and elective abortion in women over 35: utilization and relative impact on the birth prevalence of Down syndrome in Washington State.

We have examined the relative contributions of a population-based antenatal detection program and unrestricted elective abortion used by women 35 years old and over in Washington State and King County for 1976--1977 to the declining birth prevalence of Down syndrome (DS). The amniocentesis/live birth (LB) ratio for women 35 years old and over was 20.5/100 LBs for King County and 11.7/100 LBs for Washington State in 1977. For the state in 1976, abortion ratios/1,000 LBs were 571 for women 35 to 39 years old and 1,096 for women 40 years old and over. Based on the total Washington State data, elective abortion has a greater impact on averting DS births than does the antenatal detection program. The impact of elective abortion appears to be related to its disproportionate use by women 35 years old and over compared to their contribution to the population of of LBs.