Prenatal Analysis

• Things the genetic counsellor should consider in prenatal testing

  • Does the couple wish for prenatal diagnosis?
  • Could a termination of pregnancy be at all acceptable for them?
  • Is the disorder sufficiently severe?
  • Is treatment ineffective?
  • Is there a significant genetic risk to the pregnancy?
  • Is an accurate prenatal test available?
• Prenatal screening

  Identifies those at high risk of a disorder. The high-risk group can then be offered prenatal diagnosis.
• Prenatal diagnosis


  Establishes whether or not the aberration is definitely present. The main techniques for sampling are amniocentesis and chorionic villus biopsy.
• Non-invasive prenatal sampling

  Certain prenatal tests can be done with material obtained by non-invasive methods. Positive results would have to be confirmed but these tests change the approach to screening procedures.
• Several severe developmental defects, including neural tube defects (anencephaly and open spina bifida), congenital nephrotic syndrome, and abdominal wall defect, are associated with increased levels of αfetoprotein (AFP) in both the amniotic fluid and maternal serum.
• Maternal serum AFP screening for NTDs is neither 100% sensitive nor 100% specific. As a result, around 75% of screened open spina bifida cases are detected.
• Maternal serum alpha feto protein

  Levels at 16 weeks gestation
• Threshold


  MSAFP levels of and above 2.5 MoMs = further investigations
• Aneuploidies

  Chromosomal Abnormalities
• Downs


  Trisomy 21
• Nuchal translucency test


  • The thickness (translucency) of the nuchal fold (a temporary accumulation of fluid at the back of the neck) of the foetus is measured in an ultrasound scan between 11 and 14 weeks' gestation.
  • An increased swelling (>3.5mm) is typically associated with a chromosomal abnormality (e.g., Down syndrome) and/or cardiac anomaly.
  • Not diagnostic & not specific
  • This finding will lead to detailed foetal heart scanning and, usually, invasive testing. About 80% of Down cases can be identified by this method.
• Combined test

  1. Between 11 and 14 weeks of pregnancy, a combination of the nuchal translucency measurement and a blood sample from the mother.

  2. Two serum levels are taken from the blood sample: pregnancy associated plasma protein-A (PAPP-A), and free β human chorionic gonadotropin (free β-hCG).

  3. Together with the mother's age, these are used to estimate her chances of having a pregnancy with Down, Pätau or Edwards syndrome. If the chance is higher than 1 in 150, a diagnostic test will be offered.
• Triple test

  1. Levels of three metabolites in the maternal blood are measured: α-fetoprotein (AFP), human chorionic gonadotropin (hCG), and unconjugated oestriol (uE3).

  2. Maternal blood is taken at 15-16 weeks gestation.

  3. A risk value, modified by the maternal age, is given.

  4. 60% of Down syndrome cases can be detected by this method.
• Risk of trisomy 21 increases as maternal age increases


  False positives also increase with maternal age
• Quadruple test

  1. A sample of blood is taken usually between 16-20 weeks of pregnancy. The following substances will be taken from the blood sample and measured: αfetoprotein (AFP), inhibin-A, unconjugated oestriol (uE3), and free β human chorionic gonadotropin (free βhCG) or total hCG.

  2. The results of these tests are used together with the mother's age to estimate her risk to have a pregnancy with Down syndrome. Women with a statistical chance greater than 1 in 150 are offered a diagnostic test.
• Prenatal analysis


  • Biochemical analysis
  • Chromosome analysis
  • DNA analysis: indirect analysis, quantitative PCR and quantitative fluorescent PCR (qPCR, QF-PCR), targeted DNA sequencing, high-throughput sequencing
• Non-invasive prenatal testing (NIPT)

  Analyses both the baby's and the mother's cell-free DNA. With more than 98% accuracy, NIPT is more accurate than other tests for estimating the chance that the baby has an aneuploidy. Since 2021, NIPT is offered within the national screening pathway for Down syndrome, Edwards syndrome and Pätau syndrome.
• High-throughput sequencing


  Prenatal testing using high-throughput sequencing with cell-free foetal DNA, aligning DNA fragments of genomic sequences
• Chorionic villus biopsy (CVS)

  A sample is taken from the chorion, then this is dissected under the microscope. Usually performed at 11-14 weeks of gestational age. The additional risk of causing a miscarriage is 2-3%.
• Amniocentesis

  The collected amniotic fluid can be analysed biochemically or foetal cells can be extracted and cultured. Usually performed at 14-20 weeks of gestational age. The additional risk of causing a miscarriage is 0.5-1%.
• Karyotyping


  FISH karyotyping: Amniotic fluid is cultured and fluorescent probes specific for regions of chromosomes are hybridised. Two (normal) or three (trisomy) copies of the analysed chromosome can be seen under the microscope. FISH karyotyping is 100% accurate.
• QF-PCR

  DNA is extracted from amniotic fluid and then subjected to PCR. Microsatellite markers on specific chromosomes are fluorescently labelled, amplified and then quantified using an automated system. QF-PCR is not 100% accurate, but results can be reported within 48 hours, whereas standard cytogenetic techniques take approximately 2 weeks.