Changing demographics of advanced maternal age (AMA) and the impact on the predicted incidence of Down syndrome in the United States: Implications for prenatal screening and genetic counseling.

This study documents the changes in the percentages of advanced maternal age (AMA) pregnancies in the United States and in Washington State, underlying demographic factors, the impact on the predicted incidence of Down syndrome, and its impact on Down syndrome screening. Data on births in the United States from 1933 to 2002 were obtained from publications and the website of the National Center for Health Statistics. Data for Washington State were obtained from the website of the Washington State Department of Health. Information on births at Swedish Medical Center was obtained from hospital records. The percentage of AMA pregnancies in the US was about 14% before World War II, dropped steadily to about 5% in the 1970s and rose since the 1980s to about 14% in 2002. AMA Fractions are greatest among non-Hispanic Caucasians and Asian/Pacific Islanders, women who have college education and beyond, and are married. However, since 1980, the AMA Fractions have increased across all racial/ethnic groups, educational levels, and married and unmarried women. In Washington State in 2001, the overall AMA Fraction was about 14%, but there was considerable variation in the AMA Fraction across counties. In 1980, AMA pregnancies accounted for about 25% of pregnancies with Down syndrome in the United States. In 2002, AMA pregnancies accounted for more than 50% of Down syndrome pregnancies. Given the current AMA Fraction, offering amniocentesis to women of age 35 and above would result in one in seven pregnant women undergoing amniocentesis. Based on likelihood ratios, AMA as a screening strategy for Down syndrome is significantly inferior to combined serum and sonographic screening. (c) 2005 Wiley-Liss, Inc.