Introduction Down syndrome (DS) is the most common chromosomal disorder that causes mental retardation. In Thailand, studies on DS from 1969–1978 and 1988–1999, both hospital-based, found prevalences of DS of 0.89 and 1.07 per 1,000 live births.

Objective To determine the prevalence of DS in the population-based birth defect registries in 3 provinces in southern Thailand (Songkhla, Trang, Phatthalung), and the proportion of termination of pregnancy (ToP) of DS fetuses detected by prenatal screening.

Method Data were obtained from a population-based surveillance study undertaken during 2009–2012. Entries in the birth defects registry included all live births, all stillbirths, and all ToPs following a prenatal diagnosis. Infants with clinical characteristics of DS had a chromosomal study to make a definite diagnosis.

Results Of the total 148,795 births recorded during the study period, 180 DS cases were listed, giving an average prevalence of 1.21 per 1,000 births. The median maternal age was 36.4 years with percentage of maternal age ≥35 years of 52.2% (94/180). Fifty-three cases (29.4%) diagnosed prenatally resulted in termination of pregnancy.