The only way to know for sure whether or not the fetus has a chromosomal abnormality is by having an invasive test such as chorion villus sampling (CVS) or amniocentesis. However, these tests carry a risk of miscarriage of about 1/300.
The most accurate way of estimating the risk of the fetus having Down’s Syndrome is carried out at 11-13 weeks and depends on:
After the scan, on the basis of all the above factors, the estimated risk for Down’s Syndrome and other chromosomal syndromes (trisomy 13 and 18) will be discussed with you. On the basis of this risk assessment, you can decide whether you wish to have an invasive diagnostic test. It is up to you (and your partner) to decide whether or not the risk of the fetus having a chromosomal abnormality is high enough to warrant having an invasive test. As a guideline, an invasive test is usually offered if the risk of Down’s Syndrome is 1 in 300 or above.
The risk assessment remains a screening test. Under optimal conditions, 90% of babies with Down’s syndrome are detected, with a 5% chance of a false positive result. Optimal conditions include doing the blood test done at 8-10 weeks and where the ultrasound examination is done at 12 weeks by a trained professional whose examinations are subject to quality control by an external organization.
Internationally, a blood test is under development to diagnose Down’s syndrome of the baby in DNA in a blood sample of the mother’s blood. This is currently offered as alternative to women at high risk, but might in future replace the Down’s syndrome risk assessment by means of ultrasound, maternal age and levels of free ß-hCG and PAPP-A.
Irrespective of whether or not you decide to have an invasive test, it is recommended that you have a 20 weeks ultrasound scan to check for physical abnormalities.