Discuss antenatal screening for genetic abnormalities
Explain to a newly pregnant mother about the tests for Down’s syndrome
Draw a pedigree chart for a family with an autosomal recessive condition (e.g. cystic fibrosis, sickle cell anaemia) or with an autosomal dominant condition (e.g. Huntington’s, myotonic dystrophy) and discuss the risk of the patient having an affected child
Explain genetic test results and implications, e.g. patient is a carrier for the cystic fibrosis gene
Basic inheritance patterns
Autosomal dominant
Only need one copy of abnormal gene (from either parent) to cause disease:
If a parent is affected, there is a 1 in 2 chance of the child being affected
Autosomal dominant inheritance
Autosomal recessive
Need two copies of abnormal gene (one from mother, one from father) to cause disease, and only one copy to be a carrier:
If one parent (only) is a carrier, there is a 1 in 2 chance of the child being a carrier
If one parent (only) is affected, the child will be a carrier
If one parent is affected and the other is a carrier, there is a 1 in 2 chance the child will be affected, and a 1 in 2 chance the child will be a carrier
If both parents are carriers, there is a 1 in 4 chance of the child being affected, and a 2 in 4 chance of the child being a carrier
Developmental abnormalities (not genetic), e.g. neural tube defects (spina bifida), other structural developmental defects
Chromosomal abnormalities (caused by cell division error – ‘genetic’ but not usually inherited), e.g. Down’s syndrome – risk increases with age
Antenatal screening tests
Parental blood tests
Genetic testing of mother and father can be performed to determine exact risk of baby being affected by a familial (inherited) genetic condition
If there is a significant risk to the baby, invasive testing is offered
Screening for Down’s, Edwards’ and Patau’s syndromes
Combined test: scan + blood test(10-14 weeks) – initial screen in most cases (better than quadruple test)
Blood test (10-14 weeks): ↓pregnancy-associated plasma protein, ↑βHCG
Nuchal translucency scan (11-14 weeks)
Quadruple blood test (14-20 weeks): ↓α-fetoprotein, ↓unconjugated estradiol, ↑βHCG, ↑inhibin A – initial screen if combined test missed or it wasn’t possible to obtain a nuchal translucency measurement
Non-invasive prenatal testing (≥10 weeks): a more accurate (97-99%) maternal blood test that looks at fetal DNA fragments in the maternal blood– offered on NHS to mothers with a high risk (>1 in 150) combined/quadruple test result, but also avaliable privately
Neural tube defect screening
Blood test ↑α-fetoprotein(14-20 weeks) – gives a risk value. Fetal blood from amniocentesis can also be used and is more accurate.
Anomaly scan (20 weeks) – confirms
Invasive testing for genetic condition diagnosis
Invasive testing is offered if risk screening tests indicate high risk.
These tests give a definite answer to whether the child has a certain genetic condition. Results take 1-2 weeks but rapid tests for chromosome abnormalities can be done in 3 days.
They can be performed for: babies at high risk of Down’s syndrome, familial genetic conditions above
Termination of pregnancy can be performed at any time if there is confirmed genetic abnormality, but is usually done at <24 weeks
A 24 year old female asks for advice because she has had a child with cystic fibrosis. Both her and her husband are unaffected and have no other children. She would like to become pregnant again but would like to know the risk of that child being affected and the risk they would be a carrier. She would also like to know about antenatal testing.
