When a couple is having a baby, there’s a lot to think about, including whether the infant may have a condition. For Down syndrome, there are two types of tests: One tells you the baby’s risk, and the other diagnoses Down syndrome.
Screening tests can estimate the chance that your baby may have Down syndrome, but they can’t tell you for sure if your baby has it or not. Your results should be explained to you as, “You have a 1 in 1,000 chance of having a baby with Down syndrome,” or “Your baby has a 1 in 250 chance of having Down syndrome.”
There are several screening tests for Down syndrome. All measure things in the mother’s blood.
Doctors may look at the blood test along with findings from ultrasound images of the developing baby. Certain measurements, such as extra fluid at the back of the baby’s neck, can suggest your baby might have Down syndrome.
Screening tests include:
Combined first trimester screen:This test is usually done between 9 and 13 weeks of pregnancy. It’s a blood test that measures levels of a certain pregnancy hormone and a protein made by the unborn baby and mother, along with an ultrasound. When looked at together, this screening test is about 82% to 87% accurate.
Second trimester triple screen:This blood test measures hormone and protein levels along with estriol, a hormone made by the mother and the unborn baby. It’s about 69% accurate.
Second trimester quad screen.This blood test combines the triple screen with a test that looks at another hormone made by the placenta called inhibin-A. It can accurately predict about 81% of Down syndrome cases.
Integrated screening.This includes the combined first trimester screen and the second trimester quad screen. Together, this combination can identify about 94% to 96% of cases of Down syndrome.
Another screening test appears to be more accurate than the others. Called cell-free DNA (cfDNA), it finds small fragments of fetal DNA floating in the mother’s blood and tests them. It can be done after 10 weeks of pregnancy. But these tests are still in development and can sometimes give false results.