What Happens at the First Trimester Screen

Here's what you can expect if you decide to go ahead with this new, two-part prenatal test.
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BySharon Phelan, MD
Mar 2017
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The First Trimester Screen is a new optional test used to look for chromosomal abnormalities such as Down syndrome, Trisomy 18 and Trisomy 13. The screening involves two parts: a  blood test and a special ultrasound. If you decide to have a First Trimester Screen (it’s typically done between weeks 11 and 14), a lab will draw a sample of blood from your arm, which will then be used to measure your levels of two pregnancy hormones, hCG and PAPP-A. The second part of the screening, also known as a nuchal translucency scan or an NT scan, is an ultrasound that’s used to measure the fluid beneath the skin of baby’s neck. The results of both the blood test and the NT scan—along with your age—can help your doctor calculate the odds that baby has a chromosomal abnormality.

It’s important to understand that the First Trimester Screen is a screening test, and not a diagnostic. That means it will just give you the odds that there is a problem, not actually diagnose one. If your First Trimester Screen suggests that you’re likely carrying a baby with a chromosomal abnormality, your OB may recommend additional testing, such as an amniocentesis, to confirm or rule out the diagnosis.

Some women opt out of the screening altogether, reasoning that they wouldn’t change anything even if the test suggests that baby may have an abnormality. Other women choose to go ahead with the screen. The choice is entirely yours, so discuss the risks and benefits of First Trimester Screening with your doctor before making a decision.

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