What prenatal screening is primarily used to assess for Down syndrome?

The first-trimester combined screening is a comprehensive approach that involves both blood tests and an ultrasound to assess the risk of Down syndrome, also known as trisomy 21. This screening typically occurs between 11 to 14 weeks of gestation and measures specific markers in the mother's blood, such as pregnancy-associated plasma protein-A (PAPP-A) and free beta-human chorionic gonadotropin (hCG). Along with these blood tests, the nuchal translucency of the fetus is measured via ultrasound. This combination improves the accuracy of detecting potential chromosomal abnormalities like Down syndrome, allowing healthcare providers to calculate a risk ratio for the condition.

In contrast, while other options may play a role in prenatal screening, they are not primarily focused on assessing Down syndrome. The second-trimester ultrasound may identify some physical markers of Down syndrome but is less effective at earlier detection. Amniocentesis is a diagnostic test performed later in pregnancy that can confirm Down syndrome but does not serve as a preliminary screening method. Triple marker screening is used in the second trimester and measures different markers but is less specific than the first-trimester combined screening for assessing Down syndrome risk.