The evolving national birth prevalence of Down syndrome in Taiwan. A study on the impact of second-trimester maternal serum screening

Abstract

Objectives: The aim of the present study was to determine whether the liberal use of second-trimester maternal serum screening in Taiwan started in 1994 had a measurable impact on birth prevalence of infants with Down syndrome (DS) in the past decade.

Methods: We based our study on the databases of 'National Birth Defect Registration and Notification System', 'Amniocentesis in Pregnant Women', and 'Demographic Fact Book' in Taiwan. Collected data included total registered birth number, the registered number of stillbirths, the registered numbers of live births and of DS stillbirths affected with DS, amniocentesis rates each year in pregnant women aged 35 or more, and the age distribution of pregnant women in Taiwan. The live birth rate of and total birth rate of fetuses affected with DS, and the rates of live birth and stillbirth to total birth with DS, were analyzed year by year, in order to understand the change of birth rate of infants affected with DS between 1993 and 2001. Those with isolated cleft palate (ICP) were also analyzed as internal control variable. Confidence interval of live birth rate of infants with DS under Poisson distribution was calculated. Chi-square test for trend in binomial proportions was performed to see if there is an increasing (or decreasing) trend in the proportion of incidence of fetuses affected with DS. The difference was statistically significant if a p value was <0.05.

Results: A total of 1 331 616 deliveries were collected during the study period, including 840 cases of DS confirmed by karyotyping study. A marked decrease in the live birth rates of case with DS occurred in 1994-95, from 0.63 per 1000 births to 0.23 per 1000 births. There was a crossover from more live births with DS to more stillbirths with DS during 1994 to 1996 after the implementation of second-trimester maternal serum screening for DS in 1994. In 1993, 76.9% of births diagnosed with DS were born alive, compared to 32.5% in 2001 (p < 0.001).

Conclusions: The policy of prenatal diagnosis program including amniocentesis for pregnant women aged 35 or more and the liberal application of maternal serum screening for DS in younger women was responsible for the marked decrease in the live births affected with DS in Taiwan from 1993 to 2001.