Book: Genetics for obstetricians and gynaecologists: Chapter: Genetic markers on ultrasound scan.

Prenatal diagnosis is a rapidly evolving speciality. Screening for aneuploidy begins with non-sonographic features of background risk of maternal age and past and family history. It is possible to diagnose major structural defects in the foetus using second trimester scans. Serum biochemistry markers in the early second trimester were added to increase the detection rate of aneuploidy. However, as some of these abnormalities were amenable to detection earlier in the first trimester, newer modalities were introduced. Nuchal translucency (NT) measurement was one of the main advances with regard to first trimester screening. Additional markers such as the presence of nasal bone, tricuspid regurgitation, ductus venosus and megacystis; together with first trimester serum biochemistry, further enhanced the detection rate of chromosomal abnormalities. Advances in research and technology have resulted in the availability of non-invasive prenatal testing from 10 weeks of gestation. This has facilitated the detection of the three major chromosomal aneuploidies at very early gestation. However, there are a wide range of genetic syndromes that are not confined to the main trisomies. There are specific markers on ultrasound that can be linked to specific syndromes. Hence, a structured and stepwise approach is needed to identify and reach a possible diagnosis. As anomalies are classified into malformations, deformations and disruptions, it is important to note that not all markers detected are due to genetic syndromes and not all genetic syndromes can be detected on ultrasound scan. In this chapter, we outline common structural markers and their association with main genetic syndromes.