Screening Tests

Prenatal Screening Tests

Prenatal Screening Tests Web v1

ULTRASONOGRAPHY (ultrasound scan or sonar)

  • This is a non-invasive technique, where an image of the fetus is generated through the use of sound-waves.
  • It is used to assess the structure and development of the growing baby.
  • Ultrasound is both a diagnostic and screening test

When should the test be performed?

  • Dating scan (from 4 to 8 weeks)
  • First trimester (10.5 to 14 weeks)
  • Second trimester (18 to 23 weeks)

What does it detect/measure?

  • Number of babies present.
  • Gender of the baby (16 to 20 weeks).
  • Size and position of baby.
  • Heart beat of the baby.
  • Amount of amniotic fluid.
  • Blood flow from the mother to the baby.
  • Structural and non-structural markers (risk indicators).

      Non-structural Markers

Non-structural markers are features that are not definitive but are associated with an increased risk for a birth defect or genetic condition, prompting further assessment.

Nuchal translucency (clear space due to accumulation of fluid at back of baby’s neck)

  • Measured from (10.5 to 14 weeks)
  • Increased risk if measurement greater than 2.5mm

Nuchal fold 

  • Measured during second trimester
  • Increased risk greater 5mm (<20 weeks) and 6 mm (>20 weeks)

      Structural assessment (major and minor structural abnormalities)

Structural markers (major and minor markers) are abnormalities that effect function or that form part of a diagnosis for a genetic conditions or birth defect e.g. neural tube defect (spina bifida).

  • Major markers: Nasal bone, long bones (thigh and upper arm bones), feet, hands, skull, spine
  • Minor markers: Appearance of bowel, heart and brain. 

What are the benefits of ultrasound?

  • The test is non-invasive and is safe for mother and baby (no risks or side-effects).
  • A normal ultrasound may result in reassurance about your pregnancy.

What are the limitations of ultrasound?

  • Affected by the characteristics of the mother, such as ethnicity, weight, timing of conception.
  • Ultrasound scan operator bias.



  • Maternal Serum Screening (MSS) measures the level of certain products (proteins) in your blood when you’re pregnant.  
  • The proteins are: Papp-A, β-hCG, estriol, inhibin-A and alpha fetoprotein (AFP)
  • These products are present in all pregnancies but their levels indicate a risk of particular birth defect and/or chromosomal abnormalities.

When should the test be performed, and what products are tested?

       First trimester – 9 to 13.6 weeks (optimally 9 to 10 weeks)

  • Papp-A, β-hCG

       Second trimester – 16 to 20 weeks

  • Triple test: β-hCG, estriol, and alpha fetoprotein (AFP)
  • Quadruple test: β-hCG, estriol, and alpha fetoprotein (AFP) and inhibin-A

What risks does a maternal serum screen detect?

  • Chromosomal abnormalities such as trisomy 21 (Down syndrome), Edwards syndrome (Trisomy 18) and Patau syndrome (Trisomy 13).
  • Neural tube defect such as spina bifida.

What are the limitations of the first trimester maternal serum screen?

  • Affected by the characteristics of the mother, such as ethnicity, weight and method of conception
  • The test is known to have a 5% false positive result - meaning that 1 in 20 women will be told that their pregnancy has an increased risk but there is no additional risk.
  • The results are not conclusive as it only detects about 65% of pregnancies with a chromosomal abnormality.  The accuracy can be increased by including the result of other screening tests, such as an ultrasound scan, this is called a combined first trimester screen.

What do the results from the maternal serum screen test mean?

The results are given as a risk that the baby may have a chromosomal abnormality or birth defect in relation to the maternal age risk.

What are the advantages of maternal serum screening test?

  • The test is non-invasive and is safe for mother and baby (no risks or side-effects).
  • A normal result may provide reassurance about the pregnancy.
  • No side-effects.
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