Prenatal Screening for Chromosomal Defects

Chromosomal defects are a significant cause of perinatal death and childhood disability, occurring in 3.6–6.0 per 1,000 births in unscreened populations. Common chromosomal defects include trisomy 21, 18, 13, triploidy, and sex chromosome abnormalities. Screening for these defects began in the mid-60s with the advent of amniocentesis, and various methods have since been developed to improve screening performance. Initial screening was based solely on maternal and gestational age, a method incorporated later into all subsequent screening methods giving an a-priori background risk. This a-priori background risk, that is further refined by maternal serum biochemistry, results on ultrasound examinations, and most recently, results of non-invasive prenatal testing by cell-free-DNA in maternal blood. This paper will describe methods of screening for all chromosomal defects and their performance. Unlike most reviews, this paper covers not only screening tests for Down syndrome, but also screening methods for the other most common and less common chromosomal defects.