Final answer:
During the first trimester, Down syndrome can be screened for using a combination of ultrasound and blood tests. The ultrasound measures the thickness of the fluid buildup at the back of the fetus' neck, while the blood test measures the levels of certain substances in the mother's blood. Further diagnostic tests may be recommended if the results indicate a higher risk.
Step-by-step explanation:
During the first trimester, Down syndrome can be screened for using a combination of ultrasound and blood tests. The ultrasound, known as nuchal translucency (NT) screening, measures the thickness of the fluid buildup at the back of the fetus' neck. An increased thickness may indicate a higher risk of Down syndrome. The blood test, called a triple or quadruple screen, measures the levels of certain substances in the mother's blood that are associated with Down syndrome. If the results of these tests indicate a higher risk, further diagnostic tests such as amniocentesis or chorionic villus sampling may be recommended to confirm the diagnosis.