2.4

• Antenatal screening

  Identifies the risk of a disorder in a fetus or embryo before it is born, so that further tests and a prenatal diagnosis can be offered
• Antenatal screening procedures

  • Ultrasound imaging scan
  • Blood and urine tests
  • Amniocentesis
  • Chorionic villus sampling (CVS)
• Ultrasound imaging scan

  1. Dating scans (8-14 weeks) determine pregnancy stage and due date

  2. Anomaly scans (18-20 weeks) may detect serious physical abnormalities in the fetus
• Blood and urine tests (biochemical tests)

  Carried out throughout pregnancy to monitor concentrations of marker chemicals, can detect gestational diabetes, Down's syndrome, neural tube defects
• Measuring a chemical at the wrong time could lead to a false positive result
• An atypical chemical concentration can lead to diagnostic testing to determine if the fetus has a medical condition
• Amniocentesis

  A sample of amniotic fluid containing fetal cells is taken via a needle and syringe
• Chorionic Villus Sampling (CVS)

  A sample of cells are taken directly from the placenta using a needle
• Karyotyping

  Cells obtained are grown in culture, chromosomes become visible and can be analysed to show the individual's chromosomes arranged in homologous pairs
• Diagnostic tests like amniocentesis and chorionic villus sampling (CVS) can be carried to confirm presence of conditions like Down syndrome
• CVS

  Can be carried out earlier in pregnancy than amniocentesis, although it has a higher risk of miscarriage
• Why is antenatal screening carried out

  To identify the risk or diagnose a condition in a fetus before it is born
• Dating scan

  Carried out by ultrasound to establish the stage of pregnancy and due date of birth, takes place between 8-14 weeks
• Anomaly scan

  Detects serious physical abnormalities in the fetus, takes place between 18-20 weeks
• Blood and urine tests

  To monitor the concentrations of chemical markers
• A false positive result can happen if a biochemical test is carried out at the wrong time
• CVS

  Takes samples of cells from the placenta, they are cultured in a lab and then a karyotype is produced. It is carried out early in pregnancy but has a high risk of miscarriage
• Amniocentesis

  Takes samples of cells from the amniotic fluid, they are cultured in a lab and then a karyotype is produced. It is carried out later in pregnancy but has a lower risk of miscarriage compared with CVS
• Karyotype

  Shows pairs of homologous chromosomes and can be used to detect chromosomal abnormalities
• Pedigree chart


  Shows how a genetic trait is passed through several generations, useful as a predictor of how likely a person is to develop such genetic traits
• Autosomal and sex chromosomes

  Normal human cells have 46 chromosomes in 22 homologous pairs of autosomes and 1 pair of sex chromosomes. Autosomal disorders affect body cells, sex-linked disorders affect sex cells.
• Autosomal Recessive Disorders

  • Very rarely expressed, usually skips a generation, affects males and females equally, requires two copies of abnormal allele, can be carried by heterozygous individuals, can result from two unaffected carrier parents. Examples: cystic fibrosis, albinism.
• Autosomal Dominant Disorders


  • Only one copy of affected gene has to be inherited, affected individuals occur in every generation, affects males and females equally, means anyone affected has an affected parent, can be heterozygous or homozygous for the dominant allele. Example: Huntington's Disease.
• Autosomal Incomplete dominance

  Neither version of the allele for the gene is dominant over the other, so when an affected allele is inherited with a normal allele a milder form of the condition occurs. Example: sickle cell anaemia.
• Sex Linked Inheritance

  Genes on the sex chromosomes are affected (on X chromosome), affects many more males than females as males only need one copy of the defective allele, fathers cannot pass the defective allele to their sons. Examples: haemophilia, Duchenne muscular dystrophy.
• Rules for inheritance

  • If it occurs in every generation it is dominant
  • If it skips a generation it is recessive
  • If many more males are affected than females it is probably sex linked
  • If roughly equal numbers of males and females are affected it is autosomal
  • Autosomal mutations occur on chromosomes 1-22, sex linked mutations occur on the X chromosome
• Post natal screening


  Diagnostic testing to check for abnormalities in a baby after it is born