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Screening for Chromosomal Anomalies

Why is screening for chromosomal anomalies needed?

The vast majority of pregnancies result in the delivery of a healthy baby, however a small percentage may have problems affecting the baby’s development. One of the causes for these problems included chromosomal anomalies. Chromosomal anomalies are diseases or syndromes that develop as a result of an abnormality in the number or structure of the chromosomes that make up the DNA of the developing fetus. One of the most common chromosomal anomalies is Down syndrome.

Screening may be offered for women at risk of developing pregnancies with chromosomal anomalies. Some of these risks include advanced maternal age or having a previous child with a chromosomal anomaly. The expectant mother may choose not to have screening despite being offered prenatal screening.

Screening may help prepare the parents with regards to how to look after a child with special needs and also allows early prediction of possible pregnancy complications.  A screening test can only indicate whether a pregnancy is at a high or low risk for having a baby with a chromosomal anomaly. If the test indicates a high risk, parents may choose to have a diagnostic test later which will definitively say whether there are any chromosomal anomalies present in the unborn baby.

  • Incidence of common types of chromosomal anomalies

    • Down Syndrome (Trisomy 21)
      • Overall risk is 1 in 1000
      • The incidence increases with maternal age with 1:84 at age 40 and 1:44 at age 50.
      • In Malaysia, a study done showed the incidence is around 1 in 860 to 1 in 981. A more recent study in 2007 shows the incidence at 1 in 1429, with up to 1 in 323 in women aged 35 years and older.
      • May live up to 60 years old with good medical attention and early intervention for possible health complications.
    • Edwards Syndrome (Trisomy 18)
      • 3 in 10,000 births
      • Most babies will die before or shortly after birth
    • Patau Syndrome (Trisomy 13)
      • 2 in 10,000
      • Most babies will die before or shortly after birth
  • Implications of chromosomal anomalies

    • Health
      • Babies born with Down syndrome will require extra medical attention and care. There are many health issues faced by babies with Down syndrome including intellectual disability, heart disease, hearing problems, delayed development, epilepsy and an increased risk for developing other diseases including certain cancers and also Alzheimer’s.
      • These potential problems will need to be detected early and treated if possible to optimize quality of life.
      • There will be issues experienced by the carers or parents of children with Down syndrome as well, including increased risk of depression, financial burden and occasionally marital breakdown.
    • Economic: There will be a significant financial burden in caring for a child with special needs with frequent medical checks, equipment, surgeries as well as finding special needs schools or programmes.

Types of screening

  • First trimester:
    • Ultrasound: Nuchal translucency scan measures the thickness of fluid under the skin of the neck of the fetus.  This has a detection rate of anomalies of up to 77%. This may be combined with biochemical screening (blood test) as well. Together, this is known as the combined test. The combined test has a detection rate of up to 85-90%. There is 5% chance that a positive result is wrong. 
    • Biochemical screening (Blood test): PAPP-A (pregnancy associated plasma protein A) and beta hCG (human chorionic gonadotrophin). These tests are done in combination with an ultrasound for nuchal translucency as mentioned above.
  • Second trimester:
    • “Quadruple” test – includes measuring levels of alpha fetoprotein, human chorionic gonadotrophin, estriol and inhibin. These tests can detect up to 81% of cases of babies with Down syndrome. These tests can aid in detecting chromosomal anomalies and also problems associated with the spine and brain such as spina bifida and anencephaly.
    • A detailed ultrasound scan at 20-24 weeks may show any structural anomalies which may or may not be associated with chromosomal anomalies. If combined with the quadruple test, the detection rate for Down syndrome can approach 90%.
  • Newer screening methods: NIPT (Non-Invasive Prenatal Testing)
    • As the name suggests, this is a non invasive test where maternal blood is taken and examined for fetal DNA. Fetal DNA may be present in maternal circulation as early as 6-7 weeks of gestation. There is a small risk of about 2% for a wrong positive result from this test.

Prenatal diagnosis

If the screening tests mentioned above indicate a high risk of the unborn baby having a chromosomal anomaly, a diagnostic test may then be performed. Some of the tests available include:

  • Chorionic villus sampling
    • An invasive test taking a sample of placental tissue either through the abdomen or from below, through the cervix.
    • This test is performed between 10-13 weeks gestation.
    • May result in pregnancy loss (miscarriage) of 0.6-4.6%.
    • Has a diagnostic rate of almost 98%
  • Amniocentesis
    • An invasive test sampling the amniotic fluid using a needle through the abdomen.
    • Typically done between 14-20 weeks gestation.
    • Has a pregnancy loss rate of up to 1%.
    • Has a diagnostic rate of more than 99%.
  • Fetal blood sampling or cordocentesis
    • An invasive test where a needle is advance with ultrasound guidance to the umbilical cord and blood is withdrawn.
    • Usually performed later at 18-24 weeks
    • May be performed if amniocentesis/chorionic villus sampling is not successful.
    • 1-2% risk of pregnancy loss

Benefits of screening

    • Prepare parents for having a child with a possible chromosomal anomaly
    • Improve quality of life through early risk identification
    • Reduce the risk of developing later complications – i.e. early intervention

Limitation of screening

      • Routine screening tests will place a huge financial burden and therefore currently in Malaysia, it is offered to women who request it or for those women with advanced maternal age
      • The screening tests cannot say with a hundred percent accuracy whether there is a  chromosomal anomaly i.e.:
        • False negative: Screening test is negative for a chromosomal anomaly but there actually is a chromosomal anomaly
        • False positive: Screening test is positive for a chromosomal anomaly but there is actually no chromosomal anomaly.

References

Last Reviewed : 23 August 2019
Writer : Dr. Shari b. Nor
Accreditor : Dr. Hj. Mohamed Hatta b. Mohamed Tarmizi
Reviewer : Dr. Rafaie bin Amin