If you have prenatal screening done and the tests find abnormal results, further tests will be ordered. When the result of the tests is positive, that only means that a change in a chromosome, protein, or gene was detected, and does not necessarily indicate a problem. False positives are also not uncommon.
If the results of further testing come back positive, you will be given the option of carrying the pregnancy to term or aborting the fetus. If you choose to continue the pregnancy, knowing what problems your baby will face ahead of time will give you and your medical team time to prepare for the birth and the special needs of the child after it is born. Counseling will also be made available to you, no matter which option you choose.
Accuracy of prenatal screening results vary, depending on which test has been performed, and how many tests have been done. A U.S. study looked at the results of 38,167 women who underwent genetic screening using a blood test and ultrasound during their first trimester and a blood test during their second trimester, and followed up after the births. The first trimester test, done at 11 weeks, accurately identified 87% of the Downs syndrome cases. The latter test caught 81%.
When both tests are performed, 95% of cases were accurately identified. Still, this means that in five per cent of cases the test was resulting in a false positive.