Prenatal diagnosis is important in detecting and preventing
genetic disease. Significant advances since the mid-1980s have
been the development of chorionic villus sampling procedures
in the first trimester and the application of recombinant DNA
techniques to the diagnosis of many mendelian disorders.
Techniques for undertaking diagnosis on single cells has more
recently made preimplantation diagnosis of some genetic
disorders possible. Various prenatal procedures are available,
generally being performed between 10 and 20 weeks’ gestation.
Having prenatal tests and waiting for results is stressful for
couples. They must be supported during this time and given
full explanation of results as soon as possible. Most tertiary
centres have developed fetal management teams consisting of
obstetricians, midwives, radiologists, neonatologists, paediatric
surgeons, clinical geneticists and counsellors, to provide
integrated services for couples in whom prenatal tests detect an
abnormality.
Saturday, April 11, 2009
Indications for prenatal diagnosis
Prenatal diagnosis occasionally allows prenatal treatment to be
instituted but is generally performed to permit termination of
pregnancy when a fetal abnormality is detected, or to reassure
parents when a fetus is unaffected. Since an abnormal result on
prenatal testing may lead to termination this course of action
must be acceptable to the couple. Careful assessment of their
attitudes is important, and all couples who elect for
termination following an abnormal test result need counselling
and psychological support afterwards. Couples who would not
contemplate termination may still request a prenatal diagnosis
to help them to prepare for the outcome of the pregnancy, and
these requests should not be dismissed. The risk of the disorder
occurring and its severity influence a couple’s decision to
embark on testing, as does the accuracy, timing and safety of
the procedure itself.
instituted but is generally performed to permit termination of
pregnancy when a fetal abnormality is detected, or to reassure
parents when a fetus is unaffected. Since an abnormal result on
prenatal testing may lead to termination this course of action
must be acceptable to the couple. Careful assessment of their
attitudes is important, and all couples who elect for
termination following an abnormal test result need counselling
and psychological support afterwards. Couples who would not
contemplate termination may still request a prenatal diagnosis
to help them to prepare for the outcome of the pregnancy, and
these requests should not be dismissed. The risk of the disorder
occurring and its severity influence a couple’s decision to
embark on testing, as does the accuracy, timing and safety of
the procedure itself.
Identifying risk
Pregnancies at risk of fetal abnormality may be identified in
various ways. A pregnancy may be at increased risk of Down
syndrome or other chromosomal abnormality because the
couple already have an affected child, because of abnormal
results of biochemical screening, or because of advanced
maternal age. The actual risk is usually low, but prenatal testing
is often appropriate, since this allows most pregnancies to
continue with less anxiety. There is a higher risk of a
chromosomal abnormality in the fetus when one of the parents
is known to carry a familial chromosome translocation or when
congenital abnormalities have been identified by prenatal
ultrasound scanning. In other families, a high risk of a single
gene disorder may have been identified through the birth of an
affected relative. Couples from certain ethnic groups, whose
pregnancies are at high risk of particular autosomal recessive
disorders, such as the haemoglobinopathies or Tay–Sachs
disease, can be identified before the birth of an affected child
by population screening programmes. Screening for carriers of
cystic fibrosis is also possible, but not generally undertaken on a
population basis. In many mendelian disorders, particularly
autosomal dominant disorders of late onset and X linked
recessive disorders, family studies are needed to assess the risk
to the pregnancy and to determine the feasibility of prenatal
various ways. A pregnancy may be at increased risk of Down
syndrome or other chromosomal abnormality because the
couple already have an affected child, because of abnormal
results of biochemical screening, or because of advanced
maternal age. The actual risk is usually low, but prenatal testing
is often appropriate, since this allows most pregnancies to
continue with less anxiety. There is a higher risk of a
chromosomal abnormality in the fetus when one of the parents
is known to carry a familial chromosome translocation or when
congenital abnormalities have been identified by prenatal
ultrasound scanning. In other families, a high risk of a single
gene disorder may have been identified through the birth of an
affected relative. Couples from certain ethnic groups, whose
pregnancies are at high risk of particular autosomal recessive
disorders, such as the haemoglobinopathies or Tay–Sachs
disease, can be identified before the birth of an affected child
by population screening programmes. Screening for carriers of
cystic fibrosis is also possible, but not generally undertaken on a
population basis. In many mendelian disorders, particularly
autosomal dominant disorders of late onset and X linked
recessive disorders, family studies are needed to assess the risk
to the pregnancy and to determine the feasibility of prenatal
Severity of the disorder
Several important factors must be carefully considered before
prenatal testing, one of which is the severity of the disorder. For
many genetic diseases this is beyond doubt; some disorders lead
inevitably to stillbirth or death in infancy or childhood.
Requests for prenatal diagnosis in these situations are high.
The decision to terminate an affected pregnancy may be easier
to make if there is no chance of the baby having prolonged
survival. Equally important, however, are conditions that result
in children surviving with severe, multiple, and often
progressive, physical and mental handicaps, such as Down
syndrome, neural tube defects, muscular dystrophy and many
of the multiple congenital malformation syndromes. Again,
most couples are reluctant to embark upon another pregnancy
in these cases without prenatal diagnosis. Termination of
pregnancy is not always the consequence of an abnormal
prenatal test result. Some couples wish to know whether their
baby is affected so that they can prepare themselves for the
birth and care of an affected child.
prenatal testing, one of which is the severity of the disorder. For
many genetic diseases this is beyond doubt; some disorders lead
inevitably to stillbirth or death in infancy or childhood.
Requests for prenatal diagnosis in these situations are high.
The decision to terminate an affected pregnancy may be easier
to make if there is no chance of the baby having prolonged
survival. Equally important, however, are conditions that result
in children surviving with severe, multiple, and often
progressive, physical and mental handicaps, such as Down
syndrome, neural tube defects, muscular dystrophy and many
of the multiple congenital malformation syndromes. Again,
most couples are reluctant to embark upon another pregnancy
in these cases without prenatal diagnosis. Termination of
pregnancy is not always the consequence of an abnormal
prenatal test result. Some couples wish to know whether their
baby is affected so that they can prepare themselves for the
birth and care of an affected child.
Treatment for the disorder
It is also important to consider the availability of treatment for
conditions amenable to prenatal diagnosis. When treatment is
effective, termination may not be appropriate and invasive
prenatal tests are generally not indicated, unless early diagnosis
permits more rapid institution of treatment resulting in a better
prognosis. Phenylketonuria, for example, can be treated
effectively after diagnosis in the neonatal period, and prenatal
diagnosis, although possible for parents who already have an
affected child, may be inappropriate. Postnatal treatment for
congenital adrenal hyperplasia due to 21-hydroxylase deficiency
is also available and some couples will choose not to terminate
affected pregnancies. However, in this condition, affected
female fetuses become masculinised during pregnancy and
have ambiguous genitalia at birth requiring reconstructive
surgery. This virilisation can be prevented by starting treatment
with steroids in the first trimester of pregnancy. Because of this,
it may be appropriate to undertake prenatal tests to identify
those pregnancies where treatment needs to continue and
those where it can be safely discontinued. Prenatal diagnosis by
non-invasive ultrasound scanning of major congenital
malformations amenable to surgical correction is also
important, as it allows the baby to be delivered in a unit with
facilities for neonatal surgery and intensive care.
conditions amenable to prenatal diagnosis. When treatment is
effective, termination may not be appropriate and invasive
prenatal tests are generally not indicated, unless early diagnosis
permits more rapid institution of treatment resulting in a better
prognosis. Phenylketonuria, for example, can be treated
effectively after diagnosis in the neonatal period, and prenatal
diagnosis, although possible for parents who already have an
affected child, may be inappropriate. Postnatal treatment for
congenital adrenal hyperplasia due to 21-hydroxylase deficiency
is also available and some couples will choose not to terminate
affected pregnancies. However, in this condition, affected
female fetuses become masculinised during pregnancy and
have ambiguous genitalia at birth requiring reconstructive
surgery. This virilisation can be prevented by starting treatment
with steroids in the first trimester of pregnancy. Because of this,
it may be appropriate to undertake prenatal tests to identify
those pregnancies where treatment needs to continue and
those where it can be safely discontinued. Prenatal diagnosis by
non-invasive ultrasound scanning of major congenital
malformations amenable to surgical correction is also
important, as it allows the baby to be delivered in a unit with
facilities for neonatal surgery and intensive care.
Test reliability
A prenatal test must be sufficiently reliable to permit decisions
to be made once results are available. Some conditions can be
diagnosed with certainty, others cannot, and it is important that
couples understand the accuracy and limitations of any tests
being undertaken. Chromosomal analysis usually provides
results that are easily interpreted. Occasionally there may be
difficulties, because of mosaicism or the detection of an
unusual abnormality. In some cases, an abnormality other than
the one being tested for will be identified, for example a sex
chromosomal abnormality may be detected in a pregnancy
being tested for Down syndrome. For many mendelian
disorders biochemical tests or direct mutation analysis is
possible. The biochemical abnormality or the presence of a
mutation in an affected person or obligate carrier in the family
needs to be confirmed prior to prenatal testing. Once this has
been done, prenatal diagnosis or exclusion of these conditions
is highly accurate. In other inherited disorders, neither
biochemical analysis nor direct mutation testing is possible.
DNA analysis using linked markers may enable a quantified risk
to be given rather than an absolute result.
to be made once results are available. Some conditions can be
diagnosed with certainty, others cannot, and it is important that
couples understand the accuracy and limitations of any tests
being undertaken. Chromosomal analysis usually provides
results that are easily interpreted. Occasionally there may be
difficulties, because of mosaicism or the detection of an
unusual abnormality. In some cases, an abnormality other than
the one being tested for will be identified, for example a sex
chromosomal abnormality may be detected in a pregnancy
being tested for Down syndrome. For many mendelian
disorders biochemical tests or direct mutation analysis is
possible. The biochemical abnormality or the presence of a
mutation in an affected person or obligate carrier in the family
needs to be confirmed prior to prenatal testing. Once this has
been done, prenatal diagnosis or exclusion of these conditions
is highly accurate. In other inherited disorders, neither
biochemical analysis nor direct mutation testing is possible.
DNA analysis using linked markers may enable a quantified risk
to be given rather than an absolute result.
Screening tests
Screening tests aim to detect common abnormalities in
pregnancies that are individually at low risk and provide
reassurance in most cases. There is widespread application of
routine screening tests for Down syndrome and neural tube
defects by biochemical testing and for fetal abnormality by
ultrasound scanning. Most couples will have little knowledge of
the disorders being tested for and will not be anticipating an
abnormal outcome at the time of testing, unlike couples
undergoing specific tests for a previously recognised risk of a
particular disorder. It is very important to provide information
before screening so that couples know what is being tested for
and appreciate the implications of an abnormal result, so that
they can make an informed decision about having the tests.
When abnormalities are detected, arrangements need to be
made to give the results in an appropriate setting, providing
sufficient information for the couple to make fully informed
decisions, with continuing support from clinical staff who have
experience in dealing with these situations.
pregnancies that are individually at low risk and provide
reassurance in most cases. There is widespread application of
routine screening tests for Down syndrome and neural tube
defects by biochemical testing and for fetal abnormality by
ultrasound scanning. Most couples will have little knowledge of
the disorders being tested for and will not be anticipating an
abnormal outcome at the time of testing, unlike couples
undergoing specific tests for a previously recognised risk of a
particular disorder. It is very important to provide information
before screening so that couples know what is being tested for
and appreciate the implications of an abnormal result, so that
they can make an informed decision about having the tests.
When abnormalities are detected, arrangements need to be
made to give the results in an appropriate setting, providing
sufficient information for the couple to make fully informed
decisions, with continuing support from clinical staff who have
experience in dealing with these situations.
Methods of prenatal diagnosis
Maternal serum screening
Estimation of maternal serum fetoprotein (AFP)
concentration in the second trimester is valuable in screening
for neural tube defects. A raised AFP level indicates the need
for further investigation by amniocentesis or ultrasound
scanning. In some centres amniocentesis has been replaced
largely by high resolution ultrasound scanning, which detects
over 95% of affected fetuses.
Estimation of maternal serum fetoprotein (AFP)
concentration in the second trimester is valuable in screening
for neural tube defects. A raised AFP level indicates the need
for further investigation by amniocentesis or ultrasound
scanning. In some centres amniocentesis has been replaced
largely by high resolution ultrasound scanning, which detects
over 95% of affected fetuses.
maternal serum AFP
maternal serum AFP, human
chorionic gonadotrophin (HCG) and unconjugated estriol
(uE3) in the second trimester was shown to be an effective
screening test for Down syndrome, providing a composite risk
figure taking maternal age into account. When 5% of women
were selected for diagnostic amniocentesis following serum
screening, the detection rate for Down syndrome was at least
60%, well in excess of the detection rate achieved by offering
amniocentesis on the basis of maternal age alone. Serum
screening does not provide a diagnostic test for Down
syndrome, since the results may be normal in affected
pregnancies and relatively few women with abnormal serum
screening results actually have an affected fetus. Serum
screening for Down syndrome is now in widespread use and
diagnostic amniocentesis is generally offered if the risk of Down
syndrome exceeds 1 in 250. Screening strategies include
combinations of first trimester measurement of pregnancy
associated plasma protein A(PAPP-A) and HCG, second
trimester measurement of AFP, HCG, uE3 and inhibition A and
first trimester nuchal translucency measurement.
chorionic gonadotrophin (HCG) and unconjugated estriol
(uE3) in the second trimester was shown to be an effective
screening test for Down syndrome, providing a composite risk
figure taking maternal age into account. When 5% of women
were selected for diagnostic amniocentesis following serum
screening, the detection rate for Down syndrome was at least
60%, well in excess of the detection rate achieved by offering
amniocentesis on the basis of maternal age alone. Serum
screening does not provide a diagnostic test for Down
syndrome, since the results may be normal in affected
pregnancies and relatively few women with abnormal serum
screening results actually have an affected fetus. Serum
screening for Down syndrome is now in widespread use and
diagnostic amniocentesis is generally offered if the risk of Down
syndrome exceeds 1 in 250. Screening strategies include
combinations of first trimester measurement of pregnancy
associated plasma protein A(PAPP-A) and HCG, second
trimester measurement of AFP, HCG, uE3 and inhibition A and
first trimester nuchal translucency measurement.
The isolation of circulating fetal cells
The isolation of circulating fetal cells, such as nucleated red
cells and trophoblasts from maternal blood offers a potential
method for detecting genetic disorders in the fetus by a noninvasive
procedure. This method could play an important role
in prenatal screening for aneuploidy in the fetus, either as an
independent test, or more likely, in conjunction with other tests
such as ultrasonography and biochemical screening.
cells and trophoblasts from maternal blood offers a potential
method for detecting genetic disorders in the fetus by a noninvasive
procedure. This method could play an important role
in prenatal screening for aneuploidy in the fetus, either as an
independent test, or more likely, in conjunction with other tests
such as ultrasonography and biochemical screening.
Ultrasonography
Obstetric indications for ultrasonography are well established
and include confirmation of viable pregnancy, assessment of
gestational age, localisation of the placenta, assessment of
amniotic fluid volume and monitoring of fetal growth.
Ultrasonography is an integral part of amniocentesis, chorionic
villus sampling and fetal blood sampling, and provides
evaluation of fetal anatomy during the second and third
trimesters.
and include confirmation of viable pregnancy, assessment of
gestational age, localisation of the placenta, assessment of
amniotic fluid volume and monitoring of fetal growth.
Ultrasonography is an integral part of amniocentesis, chorionic
villus sampling and fetal blood sampling, and provides
evaluation of fetal anatomy during the second and third
trimesters.
Disorders
Disorders such as neural tube defects, severe skeletal
dysplasias, abdominal wall defects and renal abnormalities may
all be detected by ultrasonography between 17 and 20 weeks’
gestation. Centres specialising in high resolution
ultrasonography can detect an increasing number of other
abnormalities, such as structural abnormalities of the brain,
various types of congenital heart disease, clefts of the lip and
palate and microphthalmia. For some fetal malformations the
improved resolution of high frequency ultrasound transducers
has even enabled detection during the first trimester by
transvaginal sonography. Other malformations, such as
hydrocephalus, microcephaly and duodenal atresia may not
manifest until the third trimester.
dysplasias, abdominal wall defects and renal abnormalities may
all be detected by ultrasonography between 17 and 20 weeks’
gestation. Centres specialising in high resolution
ultrasonography can detect an increasing number of other
abnormalities, such as structural abnormalities of the brain,
various types of congenital heart disease, clefts of the lip and
palate and microphthalmia. For some fetal malformations the
improved resolution of high frequency ultrasound transducers
has even enabled detection during the first trimester by
transvaginal sonography. Other malformations, such as
hydrocephalus, microcephaly and duodenal atresia may not
manifest until the third trimester.
Abnormalities
Abnormalities may be recognised during routine
scanning of pregnancies not known to be at increased risk.
In these cases it may not be possible to give a precise
prognosis. The abnormality detected, for example cleft lip
and palate may be an isolated defect with a good prognosis
or may be associated with additional abnormalities that cannot
be detected before birth in a syndrome carrying a poor
prognosis. Depending on the type of abnormality detected,
termination of pregnancy may be considered, or plans made
for the neonatal management of disorders amenable to
surgical correction.
scanning of pregnancies not known to be at increased risk.
In these cases it may not be possible to give a precise
prognosis. The abnormality detected, for example cleft lip
and palate may be an isolated defect with a good prognosis
or may be associated with additional abnormalities that cannot
be detected before birth in a syndrome carrying a poor
prognosis. Depending on the type of abnormality detected,
termination of pregnancy may be considered, or plans made
for the neonatal management of disorders amenable to
surgical correction.
congenital abnormalities
Most single congenital abnormalities follow multifactorial
inheritance and carry a low risk of recurrence, but the safety of
scanning provides an ideal method of screening subsequent
pregnancies and usually gives reassurance about the normality
of the fetus. Syndromes of multiple congenital abnormalities
may follow mendelian patterns of inheritance with high risks of
recurrence. For many of these conditions, ultrasonography is
the only available method of prenatal diagnosis.
inheritance and carry a low risk of recurrence, but the safety of
scanning provides an ideal method of screening subsequent
pregnancies and usually gives reassurance about the normality
of the fetus. Syndromes of multiple congenital abnormalities
may follow mendelian patterns of inheritance with high risks of
recurrence. For many of these conditions, ultrasonography is
the only available method of prenatal diagnosis.
Amniocentesis
Amniocentesis is a well established and widely available method
for prenatal diagnosis. It is usually performed at 15 to 16 weeks’
gestation but can be done a few weeks earlier in some cases. It
is reliable and safe, causing an increased risk of miscarriage of
around 0.5–1.0%. Amniotic fluid is aspirated directly, with or
without local anaesthesia, after localisation of the placenta by
ultrasonography. The fluid is normally clear and yellow and
contains amniotic cells that can be cultured. Contamination of
the fluid with blood usually suggests puncture of the placenta
and may hamper subsequent analysis. Discoloration of the fluid
may suggest impending fetal death.
for prenatal diagnosis. It is usually performed at 15 to 16 weeks’
gestation but can be done a few weeks earlier in some cases. It
is reliable and safe, causing an increased risk of miscarriage of
around 0.5–1.0%. Amniotic fluid is aspirated directly, with or
without local anaesthesia, after localisation of the placenta by
ultrasonography. The fluid is normally clear and yellow and
contains amniotic cells that can be cultured. Contamination of
the fluid with blood usually suggests puncture of the placenta
and may hamper subsequent analysis. Discoloration of the fluid
may suggest impending fetal death.
amniocentesis
The main indications for amniocentesis are for
chromosomal analysis of cultured amniotic cells in
pregnancies at increased risk of Down syndrome or other
chromosomal abnormalities and for estimating fetoprotein
concentration and acetylcholinesterase activity in amniotic
fluid in pregnancies at increased risk of neural tube defects,
although few amniocenteses are now done for neural tube
defects because of improved detection by ultrasonography.
In specific cases biochemical analysis of amniotic fluid or
cultured cells may be required for diagnosing inborn errors
of metabolism. Tests on amniotic fluid usually yield results
within 7–10 days, whereas those requiring cultured cells may
take around 2–4 weeks. Results may not be available until
18 weeks’ gestation or later, leading to late termination in
affected cases.
chromosomal analysis of cultured amniotic cells in
pregnancies at increased risk of Down syndrome or other
chromosomal abnormalities and for estimating fetoprotein
concentration and acetylcholinesterase activity in amniotic
fluid in pregnancies at increased risk of neural tube defects,
although few amniocenteses are now done for neural tube
defects because of improved detection by ultrasonography.
In specific cases biochemical analysis of amniotic fluid or
cultured cells may be required for diagnosing inborn errors
of metabolism. Tests on amniotic fluid usually yield results
within 7–10 days, whereas those requiring cultured cells may
take around 2–4 weeks. Results may not be available until
18 weeks’ gestation or later, leading to late termination in
affected cases.
Chorionic villus sampling
Chorionic villus sampling is a technique in which fetally derived
chorionic villus material is obtained transcervically with a
flexible catheter between 10 and 12 weeks’ gestation or by
transabdominal puncture and aspiration at any time up to
term. Both methods are performed under ultrasound
guidance, and fetal viability is checked before and after the
procedure. The risk of miscarriage related to sampling in the
first trimester in experienced hands is probably about 1–2%
higher than the rate of spontaneous abortions at this time.
chorionic villus material is obtained transcervically with a
flexible catheter between 10 and 12 weeks’ gestation or by
transabdominal puncture and aspiration at any time up to
term. Both methods are performed under ultrasound
guidance, and fetal viability is checked before and after the
procedure. The risk of miscarriage related to sampling in the
first trimester in experienced hands is probably about 1–2%
higher than the rate of spontaneous abortions at this time.
Dissection of fetal chorionic villus
Dissection of fetal chorionic villus material from maternal
decidua permits analysis of the fetal genotype. The main
indications for chorionic villus sampling include the diagnosis
of chromosomal disorders from familial translocations and an
increasing number of single gene disorders amenable to
diagnosis by biochemical or DNA analysis. The advantage of
this method of testing is the earlier timing of the procedure,
which allows the result to be available by about 12 weeks’
gestation in many cases, with earlier termination of pregnancy,
if required. These advantages have led to an increased demand
for the procedure in preference to amniocentesis, particularly
when the risk of the disorder occurring is high. If prenatal
diagnosis is to be achieved in the first trimester it is essential to
identify high risk situations and counsel couples before
pregnancy so that appropriate arrangements can be made and,
when necessary, supplementary family studies organised.
decidua permits analysis of the fetal genotype. The main
indications for chorionic villus sampling include the diagnosis
of chromosomal disorders from familial translocations and an
increasing number of single gene disorders amenable to
diagnosis by biochemical or DNA analysis. The advantage of
this method of testing is the earlier timing of the procedure,
which allows the result to be available by about 12 weeks’
gestation in many cases, with earlier termination of pregnancy,
if required. These advantages have led to an increased demand
for the procedure in preference to amniocentesis, particularly
when the risk of the disorder occurring is high. If prenatal
diagnosis is to be achieved in the first trimester it is essential to
identify high risk situations and counsel couples before
pregnancy so that appropriate arrangements can be made and,
when necessary, supplementary family studies organised.
Fetal blood and tissue sampling
Fetal blood samples can be obtained directly from the umbilical
cord under ultrasound guidance. Blood sampling enables rapid
fetal karyotyping in cases presenting late in the second
trimester. Indications for fetal blood sampling to diagnose
genetic disorders are decreasing with the increased application
of DNA analysis performed on chorionic villus material. Fetal
skin biopsy has proved effective in the prenatal diagnosis of
certain skin disorders and fetal liver biopsy has been performed
for diagnosis of ornithine transcarbamylase (otc) deficiency.
Again, the need for tissue biopsy is now largely replaced by
DNA analysis on chorionic villus material and fetoscopy for
direct visualisation of the fetus has been replaced by
ultrasonography.
cord under ultrasound guidance. Blood sampling enables rapid
fetal karyotyping in cases presenting late in the second
trimester. Indications for fetal blood sampling to diagnose
genetic disorders are decreasing with the increased application
of DNA analysis performed on chorionic villus material. Fetal
skin biopsy has proved effective in the prenatal diagnosis of
certain skin disorders and fetal liver biopsy has been performed
for diagnosis of ornithine transcarbamylase (otc) deficiency.
Again, the need for tissue biopsy is now largely replaced by
DNA analysis on chorionic villus material and fetoscopy for
direct visualisation of the fetus has been replaced by
ultrasonography.
Preimplantation genetic diagnosis
Preimplantation embryo biopsy is now technically feasible for
some genetic disorders and available in a few specialised
centres. In this method in vitro fertilisation and embryo culture
is followed by biopsy of one or two outer embryonal cells at the
6–10 cell stage of development. DNA analysis of a single cell or
chromosomal analysis by in situ hybridisation is performed so
that only embryos free of a particular genetic defect are
reimplanted. An average IVF cycle may produce 10–15 eggs,
of which five or six develop to the stage where biopsy is
possible. The reported rate of pregnancy is about 20% per
cycle and confirmatory genetic testing by chorionic villus
biopsy or amniocentesis is recommended for established
pregnancies. This method may be more acceptable to some
couples than other forms of prenatal diagnosis, but has a very
limited availability.
some genetic disorders and available in a few specialised
centres. In this method in vitro fertilisation and embryo culture
is followed by biopsy of one or two outer embryonal cells at the
6–10 cell stage of development. DNA analysis of a single cell or
chromosomal analysis by in situ hybridisation is performed so
that only embryos free of a particular genetic defect are
reimplanted. An average IVF cycle may produce 10–15 eggs,
of which five or six develop to the stage where biopsy is
possible. The reported rate of pregnancy is about 20% per
cycle and confirmatory genetic testing by chorionic villus
biopsy or amniocentesis is recommended for established
pregnancies. This method may be more acceptable to some
couples than other forms of prenatal diagnosis, but has a very
limited availability.
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