Prenatal Screening

This Screening is used to identify those pregnancies in which there is a relatively greater risk of fetal disorders as a result of chromosomal abnormalities (e.g. an atypical number of chromosomes or a structural abnormality in one or more chromosome). These are:

  • Down syndrome (Trisomy 21)
  • Edward’s syndrome (Trisomy 18)
  • Patau’s syndrome (Trisomy 13)

Trisomy is a condition in which an extra copy of a chromosome is present in the cell that may result in severe mental / physical retardation.

Down syndrome, also known as Trisomy 21, is a genetic disorder that occurs in approximately 1 of 800 live births. Down syndrome is associated with mild to moderate learning disabilities, developmental delays, characteristic facial features, and low muscle tone in early infancy. Individuals with Down syndrome also have heart defects, leukemia, early onset of Alzheimer’s disease, gastro-intestinal problems and other health issues.

Edward’s Syndrome, also known as Trisomy 18, is a genetic disorder caused by the presence of all or part of an extra 18th chromosome. It is the second most common autosomal trisomy, after Down syndrome that carries to term. Edward Syndrome occurs in 1 of 6000 live births and around 80% of those affected are female. The majority of fetuses with the syndrome die before birth. The syndrome has a very low survival rate, resulting from heart abnormalities, kidney malfunctions, and other internal organ disorders.

Patau Syndrome, also known as Trisomy 13 or Trisomy D, is a chromosomal abnormality in which a patient has an additional chromosome due to a non-disjunction of chromosome during meiosis. The extra chromosome 13 disrupts the normal course of development, causing heart and kidney defects. Patau syndrome affects somewhere between 1 in 10,000 and 1 in 21,700 live births.

Significance

All couples have approximately 2% risk of having a child with a birth defect.

Over 70% of Down syndrome infants are born to women under 35 years of age

Mother’s Age Changes of Down Syndrome
25 1 in 1250 mothers
30 1 in 840 mothers
35 1 in 356 mothers
40 1 in 94 mothers
45 1 in 24 mothers

Advantages of first trimester combined screening over Triple Test

  • Allows early detection of an increased risk of Down syndrome and other chromosomal abnormalities during pregnancy.
  • Provides more time for counseling and decision making by the couple.
  • Negative results – If the screening test does not indicate an increased risk, this allows some reassurance to the couple earlier in the pregnancy.
  • The First Trimester Screening detection rate is approximately 90 to 95% (for a false positive rate of 5%).Triple marker gives a detection rate upto 70%.
  • First trimester NT measurement is superior to second trimester serum screening for multiple pregnancies (e.g. twins).
  • National guidelines prepared in many countries such as Singapore, UK, Netherlands, Finland, Denmark, Germany, France, Spain, Vietnam, Korea favor the implementation of FTS.

Screening Model offered by Suburban Diagnostics

Both First Trimester and Second Trimester Screening Tests are performed on FMF (Fetal Medicine Foundation) accredited instruments and reagents.  The system has been fully validated and all the calculation methods, algorithms and values are supported by current published literature.

Interpretation of Prenatal Screening

Low Risk: If the risk of identified chromosomal abnormalities like Down syndrome, based on the patients age, biochemical levels & ultrasound measurements are lower than a specified limit (1:250 for Down syndrome), the patient is considered to be at a low risk of having a Down syndrome baby. However, a low risk does not necessarily rule out the possibility of a pregnancy with chromosomal abnormalities. With a low risk report the doctor may not advice further testing.

High Risk: If risk as per the screening test result is more than the specified limit (1:250), the patient is considered to be at a relatively high risk of having a Down syndrome baby. However, a high risk result from a prenatal screening test does not confirm trisomy hence it should be followed by a confirmatory test.

Confirmatory Tests

If a screening test report indicates a high risk of having a Down syndrome baby, one of the following optional tests may be advised:

Amniocentesis

Chorion Villus Sampling

Confirmatory tests are invasive and they carry a slight risk of miscarriage. Screening therefore helps ensure that invasive tests are done only for high risk patients.