Ante- and postnatal screening Flashcards
How is the health of a pregnant woman and her developing foetus monitored?
using a variety of techniques and tests
What does antenatal screening do?
identifies the risk of a disorder so that further tests and prenatal diagnosis can be offered
What can several methods of antenatal screening be used to do?
identify the risk of a foetus inheriting a genetic disorder or chromosomal abnormality
What are the 2 types of antenatal screening tests that can be carried out during a pregnant woman’s antenatal appointments?
ultrasound imaging biochemical tests (blood tests)
What is an ultrasound scanner held against?
a pregnant woman’s abdomen
What does an ultrasound scanner do?
it picks up high-frequency sounds that have bounced off the foetus and then converts these sounds into ultrasound image on the computer screen
What are the 2 types of ultrasound scans and when are they performed?
dating scan - between 8-14 weeks
anomaly scan - between 18-21 weeks
How safe are ultrasound scans?
they are completely harmless, have no known side effects on mothers or babies, and can be carried out at any stage of pregnancl
Y
What is the dating scan used to do?
determine the stage of the pregnancy (gestational age assessment) and to calculate the date when the baby is due to be born.
What are dating scans usually used in conjunction with?
biochemical tests for marker chemicals
What can be detected during the dating scans and give an example?
some abnormalities can be detected, such as neural tube defects e.g. spina bifida
What can happen if a dating scan takes place between weeks 11-14?
it can be combined with a blood test and measuring of the fluid at the back of the baby’s neck (nuchal translucency)
What is a nuchal translucency?
measuring of the fluid at the back of the baby’s neck
If a dating scan is combined with a blood test and the fluid in the tissue at nape of the baby’s neck is measured what can this accurately screen for?
down’s syndrome, however the test will only be carried out with the mother’s permission
What does an anomaly scan allow?
a check to be made for the presence of any physical abnormalities in the foetus e.g. cleft lip
What parts of the body are looked at in detail in the anomaly scan?
the baby’s bones, heart, brain, spinal cord, face, kidneys and abdomen
What will sonographer look for specifically?
for 11 conditions, some of which are very rare
When a woman is pregnant, her body naturally undergoes many physiological changes. How can these changes be monitored?
by biochemical tests
How can some medical conditions be indicated?
by the presence of certain marker chemicals in the blood and urine.
What happens to the concentration of marker chemicals throughout pregnancy?
it varies
What chemical marker is frequently analysed during pregnancy?
Human Chorionic Gonadtrophin (HCG)
What is HCG?
a hormone which is detected in the blood and urine following implantation of an embryo. It is also the basis of early pregnancy tests
Describe the levels of HCG in a normal pregnancy?
the level of HCG increases during weeks 6-10 and then decreases to a steady low level later in gestation
Describe the level of HCG in a foetus that has down’s syndrome?
the level of HCG increases during weeks 6-10 and then remains at a level
What are the 3 tests taken to monitor the health of the mother and the developing foetus on a regular basis?
thyroid function renal function liver function (also, blood pressure)
What is a false positive?
if a marker chemical was measured at the wrong time during pregnancy and found to be high. This could lead to diagnosing a fetus with a condition that they don’t have
What is false negative?
suggests a fetus doesn’t have a condition when in fact he/she does. It is for this reason that the times chosen for biochemical tests are closely synchronised with the data from ultrasound dating scans.
What is a diagnostic test?
a definitive test that will show whether a developing actually has the condition (whereas screening tests only assess the risk of a condition)
When would certain diagnostic tests be offered to pregnant women (3)
if there is evidence of a potential problem as a result of earlier routine screening tests (NT scan)
If there is a family history of a harmful genetic disorder.
The mother belongs to ‘high-risk’ category e.g. smoke, drugs, alcohol
What are the 2 common types of diagnostic tests?
Amniocentesis
Chorionic villus sampling
What do both Amniocentesis and Chorionic villus sampling rely on?
producing a karyotype
What is a karyotype?
a visual representation of an individual’s chromosome complement where the number of chromosomes,
their arrangement, size and structure can be analysed
What the analysis of a karyotype be carried out on ?
on a sample of blood, chorionic villi or other material containing cells
When preparing a karyotype, what happens to the sample of cells collected from a sample of blood, chorionic villi or other material containing cells?
they are cultured, the karyotype image is prepared by staining the chromosomes of a cell with a dye and photographing them down a microscope. The image is then analysed using a specialised computer software which arranges the chromosomes in order of size.
What can amniocentesis assess?
whether an unborn baby could develop, or has developed, an abnormality or serious genetic condition e.g. Down’s syndrome, spina bifida or sickle cell anaemia
When is amniocentesis usually carried out?
during weeks 14-16 of pregnancy if necessary and is completely voluntary
What does amniocentesis involve?
the removal of amniotic fluid which surrounds the fetus in the womb. From the fluid, foetal cells are collected and cultured before being examined under the microscope.
How can cell samples which are obtained during amniocentesis be used to assess the health of the developing baby and diagnose potential problems?
by preparing and analysing a karyotype of the baby’s chromosomes
What are the advantages of amniocentesis?
prenatal diagnosis possible
detects neural tube defects
What are the disadvantages of amniocentesis?
small risk of miscarriage (less than CVS)
gives results later in pregnancy
When can CVS take place?
as early as 8 weeks
What is CVS offered during pregnancy for?
to check for genetic disorders e.g. Down’s syndrome
What does CVS involve?
removing and testing a small sample of cells from an area of the placenta called the chorionic villi
When is CVS only offered?
if there’s a high risk a baby could have a genetic condition
How is CVS carried out?
under the continuous guidance of an ultrasound scan. This is to make sure nothing enters the amniotic sac (protective sac that cushions the baby) or touches the baby.
What happens to the sample of cells collected during CVS?
they are cultured, stained and a karyotype is produced
What are the advantages of CVS?
prenatal diagnosis possible
gives result earlier in pregnancy
What are the disadvantages of CVS?
risk of miscarriage (slightly higher than amniocentesis)
does not detect neural tube defects
what are the 4 main blood group?
A
B
AB
O
How is your blood group identified?
by antigens and antibodies present in the blood
What are antigens?
protein molecules found on the surface of red blood cells
What are antibodies?
proteins found in the plasma. Antibodies recognise foreign antigens in your body and alert your immune system to destroy them
What can receiving blood from the wrong blood group be?
life threatening
What would happen if a person with blood group A is given blood group B?
B antibodies in the plasma of the A blood will attack the B antigens on the membrane on the RBC’s of donated B blood
What is the universal blood donor?
O as it doesn’t have any A or B antigens therefore it can be given to any other blood group safely (their antibodies have nothing to attack
Which blood group can receive blood from any other blood type?
AB (because they have no antibodies hence no immune reaction
Sometimes, what other antigen do RBC’s have?
a protein known as the RhD (rhesus) antigen
During routine antenatal appointments, what is tested?
blood will be tested to determine which blood group the mother is, and whether blood is rhesus positive or rhesus negative
What happens when a rhesus-negative mother is pregnant with a rhesus-positive foetus?
a potential problem arises
When a rhesus-negative mother is pregnant with a rhesus-positive foetus, what happens?
rhesus antigens on the surfave of the fetus’s rbc’s are regarded as foreign by the mothers immune system if she comes into contact with the fetus’s blood during an event such as birth
After an event such as birth, what is the mother given and what does it do? and what is this done before?
anti-rhesus antibodies (if she isn’t sensitised) to destroy the rhesus antigens left behind by the baby. This is done before the immune system has time to respond to them
If the mothers immune system rapidly recognises the new antigen what does it produce?
a large number of IgG antibodies
What can IgG antibodies do?
cross the placenta, hence fatal cells are susceptible to destruction
When is the rhesus antigens really a problem?
2nd Rh+ child of a Rh- mother
What will all new-born babies be offered in their first 6-8 weeks?
some screening tests
What are 3 tests a new born baby is offered?
new-born physical examination
new-born hearing screening test
new-born blood spot (heel prick) test
What can early treatment do?
improve the new-borns health and prevents severe disability or even death
At present, none of the inherited disorders screened can be successfully treated, apart from?
phenylketonuria (PKU)
What is PKU?
an inborn error of metabolism which meas the body cannot properly turn food into energy
What is PKU caused by?
a genetic mutation in the human phenylalanine hydroxylase (PAH) gene
How is PKU diagnosed in newly born infants?
routinely screened for PKU by having their blood tested for the presence of excess phenylalanine
What is the treatment for PKU?
sufferer is placed on a restricted diet containing the minimum quantity of phenylalanine needed for normal growth
What is a consequence of untreated PKU?
they will develop a mental deficiency as the baby’s brain development is affected.
What does genetic screening involve?
gathering information about the occurrence of genetic conditions from an individual’s family members. This information is used to construct human pedigree charts
What does genetic counselling provide?
support, information and advice about genetic conditions
What are the X and Y chromosomes in humans?
sex chromosomes
What are autosomes?
all the other chromosomes in the genotype apart from sex chromosomes
What is meant by heterozygous?
2 alleles different
What is meant by homozygous?
2 alleles same
How can a pattern of inheritance be shown?
by collecting information about a particular characteristic form the members of the family
Give 3 reasons why genetic testing may be carried out?
diagnosing a person with a genetic condition
helping work out the risk of a person developing a particular condition
determining if a person is a carrier of a certain genetic mutation that could be inherited by any children they have
What must be remembered about family trees?
they only work out the expected risk of inheriting a genetic condition. The actual result may vary as fertilisation is a random process
What can inherited single gene disorders either be? (4)
autosomal recessive
autosomal dominant
incomplete dominance
sex linked recessive
What does an autosomal recessive disorder mean?
two copies of an abnormal gene must be present in order for the disease or trait to develop
Give an example of an autosomal recessive condition?
cystic fibrosis
How can a geneticist recognise an autosomal recessive disorder? (3)
the trait is expressed relatively rarely
the trait may skip generations
the trait is expressed in some offspring of a marriage between e.g. cousin
What does autosomal dominant inheritance mean?
Dominant conditions are expressed in individuals who have just one copy of the mutant allele
Give 2 examples of an autosomal dominant condition?
Huntington’s chorea
Acondroplasia (short-limbed dwarfism)
How can autosomal dominant inheritance be recognised? (4)
the trait appears in every generation
each sufferer has an affected parent
traits do not skip generations
males and females are affected approxequally
What is autosomal incomplete dominance?
Where 2 alleles are incompletely dominant (i.e. neither of them are dominant to the other), a heterozygous individual will have a phenotype that is a blend of the 2 homozygous types.
Give an example of a phenotype that is a blend of the 2 homozygous types?
sickle cell trait
What are the 2 alleles which the gene that codes for haemoglobin have?
H codes for normal haemoglobin
S codes for altered haemoglobin S
What genotype do unaffected indiviuals have?
HH
What genotype do heterozygous individuals have?
HS
What genotype do affected indivuals have?
SS
If a person has the genotype SS. What is their phenotype and give a description of the condition?
Sickle cell anaemia - Haemoglobin S binds with less oxygen than normal haemoglobin. RBC’s are sickle shaped and clump together. This can block blood vessels. Sufferers often die at a young age.
If a person has the genotype HS. What is their phenotype and give a description of the condition?
sickle cell trait- Individuals suffer a less serious condition called sickle cell trait, RBC’s are a normal biconcave shape and contain both normal haemoglobin and haemoglobin S
How can you recognise autosomal incomplete dominance?
fully expressed form of the disorder occurs rarely
partly expressed form occurs more frequently
sufferers of the fully expressed form have 2 parents who suffer the partly expressed form
Males and females are affected approximately equally
What is the genotype of all non-sufferers?
homozygous for one incompletely dominant allele (HH)
What is the genotype of all sufferers of the fully expressed form?
homozygous for the other incompletely dominant allele (SS)
What is the genotype of all sufferers of the partly expressed form?
heterozygous for the two alleles (HS)
What may sex-linked recessive inheritance result from?
a mutant gene located on either the X or Y (sex) chromosome. Usually such genes are found on the X chromosome
Compare the X and Y chromosome?
The X chromosome is much larger than the Y and has many genes that are not present on the Y chromosomes. These genes are said to be sex-linked
How many copies of the sex-linked gene do females and males have?
females have 2 copies and males only have 1 copy.
why do males exhibit some traits more frequently than females?
If the gene is recessive, then males only need one such recessive to have a sex-linked trait.
Give 4 examples of sex-linked recessive disorders?
Red-green colour-blindness
Male pattern baldness
Haemophilia
Duchenne Muscular Dystrophy
How can you recognise sex-linked recessive inheritance?
Many more males are affected
None of the sons of an affected male show the trait
Some grandsons of an affected male show the trait
