First Trimester Screening

using nuchal translucency (NT) and biochemistry

The evolution of prenatal screening for Down syndrome (DS) continues with the introduction of nuchal translucency (NT) measurement - along with PAPP-A, hCG, and dimeric inhibin-A - in the first trimester. This combination of markers provides early, consistent, and reliable screening for Down syndrome and trisomy 18. This test is available for women who would rather not wait for conventional second trimester screening.

At a fixed positive rate, DS detection rates for First Trimester Screening are comparable to those seen in the second trimester:

First vs. Second Trimester Screening Performance

Detection rate at 5% screen-positive rate
(maternal age included)
N= 47,053

NT plus serum markers
GA (wks) NT PAPP-A, inhA, hCG Quad
10 60% 81% 14-20
wks GA
11 67% 83%
12 69% 87%
13 69% 90%
10-13 68% 86% 81%

from First and second trimester antenatal screening for Down's syndrome: the results of the Serum, Urine and Ultrasound Screening Study (SURUSS) Health Technology Assessment 2003; Vol.7: No.11

First Trimester Screening: What you need to know

What is the basis of First Trimester Screening?

First Trimester Screening takes advantage of the association between fetal nuchal translucency and Down syndrome risk. The observation that newborns with Down syndrome have excessive skin on the back of the neck led to prenatal ultrasound examination of the width of the translucent, fluid-filled space beneath the fetal skin on the back of the neck. Multiple studies have confirmed that increased NT is associated with increased risk for Down syndrome in the fetus. In addition, several first trimester maternal serum markers are associated with Down syndrome risk. By combining fetal NT measurement with maternal biochemical measurements, an accurate Down syndrome risk can be determined.

What are the advantages of First Trimester Screening?

Patients who wish to complete the screening process during a time when the pregnancy is not yet obvious may find First Trimester Screening to be preferable to second trimester screening. While the first and second trimester detection rates for Down syndrome are comparable at a given screen positive rate, in practice positive rates for First Trimester Screening are typically higher, reflecting the higher maternal ages of women who tend to utilize this service.

What are the limitations of First Trimester Screening?

Sonographers with documented expertise in first trimester measurements of this type must conduct the NT measurements. Specialized training as well as ongoing image review are required as part of certification. Neural tube defect (NTD) risk assessment is not available in the first trimester; women with negative first trimester screen results should have additional second trimester screening (using maternal serum AFP only or ultrasound for example) to determine open NTD risk.

Is First Trimester Screening reimbursed by insurance?

Most insurers should cover First Trimester Screening to the extent that they cover prenatal ultrasound and serum screening procedures. Patients are encouraged to check with their insurance carriers if in doubt.

Screening requirements for health care professionals:

For more information about FBR's First Trimester Screening or to create an account, please contact Client Services.