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Demystifying Down Syndrome, Part 2
A blog by Regine Lim, July 11, 2014
You’re pregnant! You’ve just heard your baby’s heartbeat for the first time, and your doctor wants you back in the office in one week to screen for Down syndrome and other chromosomal abnormalities.
At this stage screening tests are non-invasive – typically, a simple blood test – meaning there’s almost no risk to your pregnancy. They provide a likelihood that your pregnancy is affected. In the next post I’ll talk about diagnostic tests, which are definitive but also more risky. If a flag is raised during screening some women will choose to get diagnostic testing.
Keep in mind that while both screening and diagnostic tests have become fairly routine, they’re also optional. Your healthcare team is there to advise you, but it’s your pregnancy, and you’re the decision-maker.
If you do choose to proceed with chromosomal screening, below is a breakdown of the tests available during your pregnancy. Keep in mind that timing is crucial:
1st trimester maternal serum screen (MSS) + nuchal translucency (NT)
This is a blood test, usually a finger-prick, in combination with an ultrasound measurement of fluid accumulation behind the baby’s neck. Your blood is tested for levels of certain risk markers, namely pregnancy-associated plasma protein-A or “PAPP-A” and human chorionic gonadotropin or “hCG.” Your age and the gestational age of the pregnancy are also used in the risk calculation.
This screening test detects the likelihood of Down syndrome and two other chromosomal abnormalities – trisomy 18 and trisomy 13. It can detect 91-95% of cases of Down syndrome and 95% of cases of trisomy 18 and 13 with a false positive rate of 5% or less.*
Timing: Between 11 weeks, 1 day and 13 weeks, 6 days gestation, only.
2nd trimester sequential screening
This is the follow-up blood test that re-measures hCG and also looks at a three new markers: alpha-fetoprotein (AFP), unconjugated estriol, and inhibin-A.
Completing this second step can help increase your detection rate for chromosomal abnormalities and also identify risk for open neural tube defects (ONTDs), one of which is spina bifida. Completed without the 1st trimester MSS + NT, it is called the Quad screen and has a lower detection rate.
Timing: Between 15 weeks and 21 weeks, 6 days gestation, only.
Can an ultrasound catch Down syndrome?
An ultrasound, like other screening tools, can be used to adjust a risk estimate, but it cannot be used to “catch” or diagnose Down syndrome and other chromosomal abnormalities.
Non-invasive cell-free fetal (cff) DNA screening
This is the newer kid on the screening block, and it has some advantages. It is not yet replacing traditional screening methods, but more clinicians are introducing this screening tool to their patients across the country. It relies on DNA that is released from placental cells (which contain the baby’s DNA). That DNA floats freely in your blood when you’re pregnant – in fact, by 10 weeks gestation it accounts for approximately 10% of the DNA in your blood. This screening test measures the relative amount of 21, 18, 13, as well as X and Y chromosomal material. If the screen detects relatively more DNA from chromosome 21 than it would expect, then the risk of Down syndrome increases.
Most of the cffDNA screens have greater than 99% sensitivity with a false positive rate for trisomies 21, 18, and 13 far below 5%. Unfortunately, this test is less reliable when the mom is overweight.
Timing: any time after 10 weeks gestation.
The screening tests that you undergo during pregnancy are entirely up to you. Get your questions answered by your healthcare team so that your decisions can be as informed as possible.
*numbers using free beta hCG