Predicted prevalence of Down syndrome live births in Japan, 1970–2006

Takeuchi et al. [2008] in this issue describe their analysis of 164 Down syndrome live births in Shimane Prefecture of Japan during 1980–1999, and show an increase of frequency from 1980– 1989 to the 1990–1999 period. This Letter deals with the demographic changes of live births from 1970 to 2006 in the entire country, and documents decreasing total live births, increasing proportions of mothers of 35 years of age or over, and increasing projected frequencies of Down syndrome live births. Data on maternal age and number of births were obtained from online databases of the National Institute of Population and Society Research [www. ipss.go.jp/], supplemented by information from the Ministry of Health, Labour and Welfare of Japan [www.mhlw. go.jp/toukei/saikin/old/k-jinkou. html]. Data for the period from 1970 to 2005 have been presented in a previous article [Kajii, 2007], while this article includes additional and more recent data. The total live births in Japan have decreased from 1.93 million in 1970 to 1.09 million in 2006, a decline of 43.5% (Table I and Fig. 1). The proportions of births to women of 35 years of age were calculated according to the method described by Resta [2005]. Their proportion was 4.7% in 1970, lowest at 3.8% in 1975, and thereafter increased steadily to 17.7% in 2006, the latest year for which data are available. The increase became steeper during 2003–2006 with annual increase rates of 1.1–1.3%. Basedonmaternal age proportion and assuming no prenatal diagnosis or termination of pregnancy, the number of projected births of Down syndrome was calculated in 5-year blocks of maternal ages (Table I). It was 2,152 in 1970, decreased to 1,646 in 1990, and increased to 1,932 in 2006. Projected frequency of Down syndrome was 1.07 per 1,000 (or 1/932) live births in 1975, and increased to 1.79 per 1,000 (or 1/566) live births in 2006. The increasing frequency of Down syndrome births due to increasing maternal age, therefore, has been cancelled out by decreasing total live births. Down syndrome births to women of 35 years or over in 1975 represented 18.2% of all Down syndrome births, and thereafter steadily increased to be 50.9% in 2006. A nation-wide survey of the status of prenatal screening tests in Japan by Sago et al. [2005] covered the 5-year period from 1998 to 2002, and was based on data collected from 30 universities, public, and commercial laboratories. Maternal serum screening tests were performed in seven laboratories in 1998 and involved 21,708 tests. In 2002, 15,627 tests were carriedout in five laboratories, a declineof 28%. The number of amniocentesis remained stable during the 5-year period with an average of 10,300 tests. Amniocentesis comprised 98.3% of invasive prenatal procedures, with chorionic villus sampling and fetal blood analyses accounting for the remainder. These figures would translate into reduction of 75 annual Down syndrome live births, allowing for elimination of 5% tests during 20 weeks of pregnancy, assuming 1% detection rate of Down syndrome, and a loss rate of 23% Down syndrome pregnancies after amniocentesis [Morris et al., 1999]. In addition to their study in Tottori Prefecture, Takeuchi et al. [2008] refer to four other studies of birth prevalence of Down syndrome individuals in various regions of Japan, and during various periods from 1973 to 2000. These studies were different in the size of population analyzed, method of data collection, and ascertainment rate; but four of the five studies in which chronological changes were analyzed showed an increase of the prevalence of Down