Test Directory

First Trimester Maternal Screen, Serum

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Updated Test Information:

Test Description
First Trimester Maternal Screen, Serum

First Trimester Maternal Screen; Combined Screen; First Trimester Screen; Ultrascreen

Test ID
General Information

Useful for:

  • Prenatal screening for Down syndrome

  • Prenatal screening for Trisomy-18

Specimen Type


Specimen Requirements

  • Preferred: Red Top

  • Acceptable: Serum Gel

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. See "Additional Information".


  • Refrigerated (preferred): 7 days

  • Frozen: 90 days

Unacceptable Specimen Conditions

Gross hemolysis


Upon receiving maternal serum screening results, all information used in the risk calculation should be reviewed for accuracy (eg, maternal date of birth, demographics, sonographic information). If any information is incorrect, the laboratory should be contacted for a recalculation of the estimated risks.

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).

  • The majority of women with a positive screen will not have a Down syndrome fetus.

  • Each center offering maternal serum screening to patients should establish a standard screening protocol that provides pre- and post-screening education and appropriate follow-up for screen-positive results.

Variables Affecting Marker Levels: 

  • All serum marker multiple of medians are adjusted for maternal weight (to account for dilution effects in heavier mothers). The estimated risk calculations and screen results are dependent on accurate information for gestation, maternal age, and weight. Inaccurate information can lead to significant alterations in the estimated risk.

  • In twin pregnancies, the risk for Down syndrome is calculated using twin-adjusted medians. Risks for triplets and higher multiples cannot be calculated.

  • Nuchal translucency (NT) measurements must be obtained from a trained and certified sonographer. NT quality indicators are monitored by the performing laboratory on a regular basis. Institutions will be contacted if there is ongoing deviation in the quality indicators.


Immunoenzymatic Assay

Estimated TAT

1 day

Testing Schedule


Test Includes

  • Results Summary

  • Down Syndrome Screen Risk Estimate

  • Down Syndrome Maternal Age Risk

  • Trisomy 18 Risk Estimate

  • PAPP-A

  • PAPP-A MoM

  • THCG

  • THCG MoM

  • NT

  • NT MoM

  • NT Twin

  • NT Twin MoM

  • Interpretation

  • Recommended Follow Up

  • *See "Test Definition: 1STT1" for complete list of codes.


12 months

CPT Code(s)


Additional Information
Minimum Sample Volume

0.75 mL

Reference Range


  • Calculated screen risks <1/230 are reported as screen negative
  • Risks ≥ 1/230 are reported as screen positive


  • Calculated screen risks <1/100 are reported as screen negative.
  • Risks ≥ 1/100 are reported as screen positive.

A numeric risk for trisomy 18 risk is provided with positive results on non-diabetic, non-twin pregnancies.
An interpretive report will be provided.

Performing Lab


LOINC Code(s)

43995-0, 50679-0, 43995-0, 49090-4, 43994-3, 48407-1, 76348-2, 32166-1, 32166-1, 49035-9, 49035-9, 49035-9, 49035-9, 49588-7, 80615-8, 48767-8, 33882-2, 21112-8, 43993-5, 29463-7, 29463-7, 44877-9, 21484-1, 64234-8, 34970-4, 11957-8, 11957-8, 11888-5, 552