Each woman will be given an estimate of her individual risk for their pregnancy. Your risk is assessed by taking into account the following:
Age of the mother – Amount of fluid behind the neck of the fetus (nuchal translucency).
Fetuses at risk of Down Syndrome tend to have a higher amount of fluid around the neck. Babies accumulate fluid at the back of their neck (nuchal) during the first trimester. As a result of this fluid, the baby’s nuchal fold increases in thickness each day of gestation until the fluid disappears around the 15th week. Researchers have established an average or “normal” thickness for each day during the three weeks you are eligible for the screening If your baby’s nuchal fold is thicker than average, he or she is considered at a higher risk for an abnormality; if it’s thinner, his or her risk is lower.
A more accurate risk assessment combines the Nuchal Translucency scan with the results from two bloods tests that measure the levels of pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotrophin (hCG) in the mother's blood. Abnormal levels of these two pregnancy-related substances may indicate a problem. Combining the scan and blood test gives a more accurate result than either the ultrasound part or the blood part of the test done on their own.
You must be at least 10 weeks gestation or more for the blood sample to be taken. The Nuchal Translucency scan cannot tell for certain if your baby has a chromosonal defect, however, it can help you decide whether or not the chance of your baby having a chromosonal problem is high enough to warrant having an invasive test such as CVS (Chorionic Villus Sampling) or Amniocentesis. However, these tests carry a slight chance of miscarriage and therefore options should be discussed with their healthcare professional. CVS is performed in the 13th week and amniocentesis around 15th or 16th week of pregnancy.