Second trimester maternal serum screening

  1. The optimal time to have this test performed is between 15 and 17 weeks, but it can be performed until 20 weeks.
  2. Second trimester maternal serum screening uses a blood test in conjunction with the maternal age, gestational age and maternal weight to calculate a risk figure for Down syndrome. This screening test may also detect pregnancies with an increased risk for trisomy 18 (see Chromosomal conditions) and neural tube defects.
  3. Maternal blood contains hormones and proteins produced by the fetus and placenta, including alpha- fetoprotein (AFP), unconjugated estriol (μE3), free ß-subunit of human chorionic gonadotrophin (free ß-hCG), and inhibin A.
  4. Levels tend to be altered in pregnancies affected by Down syndrome, trisomy 18 or neural tube defects. In Down syndrome, the levels of AFP and μE3 tend to be reduced, and free ß-hCG and inhibin A increased. In neural tube defects AFP may be increased and, in trisomy 18, levels of all these substances are decreased.
  5. The number and type of analytes used may vary between pathology services. The quadruple test measures four analytes (AFP, μE3, free ß-hCG and inhibin A), whilst the triple test measures three analytes. Detection rates are improved when four analytes are used.
  6. Using the quadruple test with ultrasound dating:
  7. 75% of fetuses with Down syndrome are detected by second trimester maternal serum screening, and approximately 5% of tests give an increased risk result. Women aged 40 and over have higher detection rates, with 95% of affected pregnancies receiving an increased risk result. At least 50% of all women in this age group will receive an increased risk result.
  8. 85% of babies with a neural tube defect will be detected using maternal serum screening alone. When combined with a detailed ultrasound, the detection rate for spina bifida can be as high as 95%, and 100% for anencephaly.
  9. Results are usually available within 24 to 48 hours of collection, but may take longer in parts of regional Australia. Women at increased risk are offered diagnostic testing. The majority of increased risk results are not due to Down syndrome and most of these babies are unaffected . The possibility of false positive results and the management options should be discussed with women prior to screening, as should the fact that this blood test cannot definitively identify babies with Down syndrome, trisomy 18, or neural tube defects.
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Arranging second trimester maternal serum screening

  1. Some States’ and Territories’ Antenatal Screening Programs receive government funding to perform this testing on public patients, and so there is no out-of-pocket cost.
  2. For private patients, the cost depends on the pathology provider.
  3. This test does not need to be repeated unless blood was collected at less than 14 weeks gestation.
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The factors that need to be entered into the risk calculation algorithm should be noted on the request form:

  1. Ultrasound-based gestation or LMP
  2. Current weight
  3. Maternal age
  4. Previous child with a chromosomal abnormality as well as previous child or close relative with a neural tube defect
  5. Date of collection
  6. If the woman has insulin-dependent diabetes
  7. And any other information requested on the form, eg ethnicity, IVF details
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