A population-based study of prevalence of Down syndrome in Southern Thailand.

Background: Down syndrome (DS) is the most common chromosomal disorder that causes mental retardation. In 2009, a population-based birth defects study was implemented in three provinces in southern Thailand. This study aimed to determine the prevalence of DS in the studied regions, and the proportion of DS fetuses detected by prenatal screening.

Methods: Data were obtained from a population-based surveillance study undertaken during 2009-2013. Entries in the birth defects registry included live births, stillbirths after 24 weeks gestational age, and terminations of pregnancy following prenatal diagnosis. Infants with clinical characteristics of DS had a chromosomal study to make a definite diagnosis.

Results: Of the total 186 393 births recorded during the study period, 226 DS cases were listed, giving a prevalence of 1.21 per 1000 births [95% confidence interval (CI) 1.05-1.37]. The median maternal age was 36.5 years with a percentage of maternal age ≥35 years of 60.6%. Seventy-seven cases (34.1% of all cases) were diagnosed prenatally and these pregnancies were terminated. The prevalence of DS per 1000 births was significantly higher in older women, from 0.47 (95% CI 0.28-0.67) in mothers aged <30 years to 0.88 (95% CI 0.59-1.17) in mothers 30-<35 years (P<0.01), and to 4.74 (95% CI 3.95-5.53) in mothers ≥35 years (P<0.001).

Conclusions: The prevalence of DS significantly increased with maternal age. About 35% of DS cases were detected prenatally and later terminated. Hence, examining only registry live births will result in an inaccurate prevalence rate of DS.