First-trimester prenatal screening for Down syndrome (trisomy 21) and trisomy 18
Testing algorithm:
Sequential maternal screening is a 2-step test, with first- and second-trimester components
A nuchal translucency (NT) measurement and blood collection in the first trimester
If the result from part 1 indicates a risk for Down syndrome that is higher than the screen cutoff, the screen is completed and a report is issued
If the results from part 1 are negative, an additional blood collection in the second trimester is required (see SEQB: Sequential Maternal Screening, Part 2, Serum)
If the second specimen is not received for sequential screening, the results are uninterpretable and no maternal risk will be provided
If collecting a secondtrimester specimen is expected to be difficult, order first-trimester screening instead (see 1STT1: First Trimester Maternal Screen, Serum)
The ultrasound and blood draw must be completed within a gestational window of 10 weeks, 0 days and 13 weeks, 6 days, which corresponds to a crown-rump length (CRL) range of 31 to 80 mm
Centrifuge and aliquot serum within 2 hours of collection
Minimum Sample Volume
0.5 mL
Required Information
Approval to send specimen for first-trimester screening is required and may take up to 5 business days to complete
Nuchal translucency (NT) measurements are only accepted from NT-certified sonographers
Do not send specimen to Mayo Clinic Laboratories if the sonographer is not NT-certified or before completing the application process
Nuchal translucency (NT) measurements must be obtained from NT-certified sonographers. NT-measurement quality indicators will be monitored on a regular basis. Sonographers will be contacted if there is ongoing deviation in the quality indicators
Incorrect or incomplete information may significantly alter results
A screen-negative result does not guarantee the absence of fetal defects. A screen-positive result does not provide a diagnosis, but indicates that further diagnostic testing should be considered (an unaffected fetus may have screen positive result for unknown reasons). In fact, given the low prevalence of Down syndrome, the majority of women with a positive screen will not have a Down syndrome fetus
In twin pregnancies, the risk for Down syndrome is approximated, using twin-adjusted medians. In cases where one twin has demised, results may be unreliable
Results are not available for triplets or higher-multiple pregnancies
Each center offering maternal serum screening to patients should establish a standard screening protocol, which provides pre- and post-screening education and appropriate follow-up for screen-positive results
