What is the routine antenatal investigation for detecting Down syndrome?

The routine antenatal investigation for detecting Down syndrome is the Nuchal translucency scan. This scan is typically performed during the first trimester of pregnancy, between 11 and 14 weeks of gestation. It involves an ultrasound measurement of the nuchal fold, which is a fluid-filled space at the back of the fetus's neck. Increased thickness of the nuchal translucency can be an indicator of chromosomal abnormalities, including Down syndrome.

This scan is part of the combined first-trimester screening which also includes maternal blood tests to analyze specific markers that can indicate the likelihood of Down syndrome. The Nuchal translucency scan is non-invasive and provides an early assessment of the risk, allowing for further diagnostic options if necessary.

While other methods such as amniocentesis and chorionic villus sampling are definitive diagnostic tests for Down syndrome, they are typically performed later in pregnancy and carry a small risk of miscarriage. Maternal serum alpha-fetoprotein testing is also a screening tool, but it is primarily used to detect neural tube defects and is less specific for Down syndrome. Thus, the Nuchal translucency scan is the most appropriate routine antenatal investigation for assessing the risk of Down syndrome.