The combined first-trimester scan is a prenatal screening test performed between 12 to 14 weeks of pregnancy. It combines ultrasound measurements and blood tests to assess the risk of Down syndrome and preeclampsia.

For Down syndrome screening, the scan measures the nuchal translucency (the fluid-filled space at the back of the baby's neck). Increased fluid thickness can indicate a higher risk of Down syndrome. Additionally, blood tests are done to measure specific hormones and proteins, such as PAPP-A and beta-HCG, which can provide further information about the risk.

Regarding preeclampsia screening, the scan may assess the blood flow in the uterine arteries as well as other markers. Abnormalities in these measurements can indicate an increased likelihood of developing preeclampsia, a condition characterized by high blood pressure and organ damage during pregnancy.

It's important to note that the combined first-trimester scan is a screening test and not a diagnostic tool. If your results indicate a high risk for Down syndrome or preeclampsia, further diagnostic tests may be recommended to confirm the findings, such as chorionic villus sampling (CVS) or amniocentesis for Down syndrome, and additional monitoring for preeclampsia.