A prenatal screen result tells you the chance of your baby having Down syndrome, trisomy 18 or an open neural tube defect.
How is my result calculated?
The levels of proteins made by the baby and the placenta are measured in your blood. If you have a nuchal translucency ultrasound, the amount of fluid normally found at the back of the baby's neck is measured. A baby with Down syndrome, trisomy 18 or an open neural tube defect may have different amounts of these proteins or fluid. These differences are used to estimate the chances for your pregnancy. Other factors that affect the results include the dating of your pregnancy, your age, weight, ethnic origin and whether it is a twin pregnancy.
As part of your screen result, you will be given a number that estimates the chance your baby has one of these conditions. This number is compared to the screen cut-off. The screen cut-off is the chance above which you will be offered diagnostic testing. If your chance is lower than the screen cut-off, this is called a screen negative result. If this number is higher than the screen cut-off, this is called a screen positive result.
What does a screen negative result mean?
If your result is screen negative, the chance that the baby has Down syndrome, trisomy 18 or an open neural tube defect is low. You can be more than 99.9% certain that your baby will not have any of these conditions.
What does a screen positive result mean?
If your result is screen positive it does not mean that your baby has the condition. In fact, most women with this result do not have a baby with one of these conditions. The result means that the chance of Down syndrome, trisomy 18 or an open neural tube defect is high enough that you will be offered diagnostic testing. Diagnostic testing tells you for sure if your baby has any of these three conditions.
What if my result is screen positive for Down syndrome or trisomy 18?
If your result is screen positive for Down syndrome or trisomy 18, your health care provider may need to confirm how far along you are in your pregnancy. If the dates are correct, you will be offered amniocentesis . Amniocentesis will tell you if your baby has either of these conditions. It is your choice whether you have the amniocentesis or not.
What if my result is screen positive for an open neural tube defect?
If your result is screen positive for an open neural tube defect, you will be offered a detailed ultrasound and an appointment with a maternal fetal medicine doctor or a genetic counsellor at one of the medical genetics clinics (in Vancouver or Victoria). If your baby does have an open neural tube defect this will usually be seen on the ultrasound scan.