Prenatal Screening and Diagnosis

In recent years the prevalence of Down syndrome has been increasing. The increase in the prevalence might be partly explained by better compilation of statistics on Down syndrome today. Also, the mean maternal age at first delivery as well as the proportion of older moth‐ ers is increasing in all western countries and the risk of Down syndrome increases with ad‐ vancing maternal age [1]. The proportion of mothers aged 35 years or older in France, Finland, Germany, Greece and United Kingdom were 15.8 %, 19.0 %, 17.0 %, 14.2 % and 17.2 % in 2001, respectively, in 2008 the proportions were 18.9 %, 18.2 %, 21.8 %, 20.9 % and 20.1 %, respectively (Eurostat). Screening for Down syndrome was first performed in 1970’s us‐ ing advanced maternal age or previous history of chromosomal abnormality. The preva‐ lence of Down syndrome at term rises from 1/1527 at the maternal age of 20 years to 1/895 at age 30 and to 1/97 at age 40 [11]. Also the gestational age affects the prevalence of Down syndrome. The estimated rate of fetal loss in Down syndrome pregnancies is 43 % between gestational week 10 and term, 23 % between gestational week 15 and term and 12 % of births are stillbirths or result in a neonatal death [12]. Therefore, the risk of Down syndrome decreases as the pregnancy progresses. Table 1 presents the prevalence of Down syndrome pregnancies in different maternal age groups according to the gestational age.