Prenatal Genetic Testing
What Is Prenatal Genetic Testing?
Prenatal genetic testing refers to genetic and chromosomal tests performed during pregnancy to assess whether an unborn baby may have certain genetic conditions or chromosomal differences. Depending on the circumstances, testing may examine the baby’s DNA or chromosomes using a sample obtained during pregnancy, or assess the chance of particular conditions through prenatal screening.
Prenatal genetic testing may be considered when there is a known genetic condition in the family, a previous pregnancy affected by a genetic or chromosomal condition, or a prenatal screening result that indicates a higher chance of a particular condition. The appropriate test depends on the specific clinical question, the stage of pregnancy and the family’s medical and genetic history.
It is important to distinguish prenatal screening from prenatal diagnostic testing. Screening estimates the chance that a baby may have a particular condition, whereas diagnostic testing is used to investigate whether a specific genetic or chromosomal condition is present. Diagnostic testing may involve procedures such as chorionic villus sampling (CVS) or amniocentesis.
Prenatal genetic testing forms part of the wider range of genetic testing available for identifying or investigating genetic conditions. The choice of test should be based on the individual pregnancy and discussed with an appropriately qualified healthcare or genetics professional.
When Is Prenatal Genetic Testing Recommended?
Prenatal genetic testing may be recommended or offered when there is an increased possibility that a baby could have a genetic or chromosomal condition. The decision depends on the individual’s pregnancy, medical and family history, previous pregnancy history and the findings of any prenatal screening or ultrasound examinations.
Prenatal genetic testing may be considered in situations such as:
- A parent or close family member has a known genetic condition or genetic variant that could be passed on to the baby.
- Previous genetic testing has identified a genetic change in one or both parents that may have implications for the pregnancy.
- A previous pregnancy or child was affected by a genetic or chromosomal condition.
- Prenatal screening indicates a higher chance of a chromosomal condition such as Down’s syndrome, Edwards’ syndrome or Patau’s syndrome.
- An ultrasound scan identifies findings that may be associated with a genetic or chromosomal condition.
- The parents are known to have a genetic or chromosomal rearrangement that may affect the pregnancy.
Testing may also be discussed when there is a particular concern about an inherited condition within the family. In these circumstances, the most appropriate test depends on the condition being considered and whether the specific genetic change is already known.
Not everyone who has one of these factors will require the same type of test. Some situations may call for prenatal screening, while others may warrant diagnostic testing such as chorionic villus sampling (CVS) or amniocentesis. The potential benefits, limitations and risks of each option should be considered with a qualified healthcare or genetics professional.
What Can Prenatal Genetic Testing Detect?
Prenatal genetic testing can be used to investigate certain genetic and chromosomal conditions during pregnancy. The conditions that can be tested for depend on the type of test, the family's medical and genetic history, and the specific condition being investigated.
| Type of Condition | Examples | What May Be Investigated |
|---|---|---|
| Chromosomal conditions | Down’s syndrome, Edwards’ syndrome and Patau’s syndrome | Changes in chromosome number or, where appropriate, other chromosomal abnormalities. |
| Single-gene conditions | Cystic fibrosis, sickle cell disease and thalassaemia | Specific genetic variants associated with a known inherited condition may be investigated. |
| Known familial genetic conditions | A genetic condition already identified in the family | Where the familial genetic variant is known, targeted testing may be used to determine whether the baby has inherited it. |
| Other genetic or chromosomal abnormalities | Conditions identified because of family history or prenatal findings | Depending on the clinical circumstances, additional genetic or chromosomal analysis may be considered. |
The range of conditions that can be investigated depends on the test being performed. Prenatal screening can identify an increased chance of certain conditions, while diagnostic testing such as chorionic villus sampling (CVS) or amniocentesis can investigate whether specific genetic or chromosomal conditions are present. A higher-chance screening result does not by itself mean that the baby has the condition.
What Is the Difference Between Prenatal Screening and Diagnostic Testing?
Prenatal screening and prenatal diagnostic testing have different purposes. Screening tests estimate the chance that a baby may have certain genetic or chromosomal conditions, while diagnostic tests are used to investigate whether a specific condition is present.
Prenatal screening may involve blood tests, ultrasound examinations or non-invasive prenatal testing (NIPT), depending on the pregnancy and the conditions being assessed. A screening result cannot confirm that a baby has or does not have a particular genetic or chromosomal condition. Instead, it can indicate whether the chance is considered lower or higher.
Diagnostic testing provides more specific information about particular genetic or chromosomal conditions. During pregnancy, diagnostic testing may involve chorionic villus sampling (CVS) or amniocentesis, in which a sample is obtained for laboratory analysis. These procedures are invasive and are generally considered when there is a specific reason to investigate a possible genetic or chromosomal condition.
A higher-chance screening result does not necessarily mean that the baby has the condition being screened for. Depending on the circumstances, further screening or diagnostic testing may be offered to provide more information. The appropriate approach depends on the screening findings, family history, previous pregnancy history and other clinical factors.
In simple terms: prenatal screening assesses the chance of a genetic or chromosomal condition, whereas prenatal diagnostic testing investigates whether a particular condition is present. Understanding this distinction can help parents make informed decisions about further testing during pregnancy.
Which Prenatal Genetic Tests Are Available?
Several types of prenatal genetic testing are available, with the most appropriate option depending on the stage of pregnancy, the condition being investigated and the individual circumstances of the pregnancy.
Prenatal screening estimates the chance of certain genetic or chromosomal conditions. It may include blood tests, ultrasound examinations and non-invasive prenatal testing (NIPT), depending on the pregnancy and the conditions being assessed.
CVS is an invasive diagnostic test that uses a small sample of placental tissue to investigate specific genetic or chromosomal conditions. It is usually performed earlier in pregnancy than amniocentesis.
Amniocentesis is an invasive diagnostic test that analyses a sample of amniotic fluid. It can be used to investigate certain genetic and chromosomal conditions and is generally performed from around 15 weeks of pregnancy.
The appropriate test depends on the individual pregnancy. Prenatal screening provides an estimate of risk, while diagnostic tests such as CVS and amniocentesis can investigate whether specific genetic or chromosomal conditions are present.
How Is Prenatal Genetic Testing Performed?
Prenatal genetic testing is performed using different methods depending on the type of test, the stage of pregnancy and the genetic or chromosomal condition being investigated. Some tests are non-invasive and use a blood sample from the pregnant woman, while diagnostic tests may require a sample obtained during pregnancy using an ultrasound-guided procedure.
For prenatal screening, a blood sample may be analysed alongside other information, such as the mother’s age and ultrasound findings, to estimate the chance of certain genetic or chromosomal conditions. Non-invasive prenatal testing (NIPT) can analyse small amounts of fetal DNA found in the mother’s bloodstream to screen for specific chromosomal conditions.
When diagnostic testing is appropriate, a sample of placental tissue or amniotic fluid may be obtained through chorionic villus sampling (CVS) or amniocentesis. The sample is then sent to a specialist laboratory, where the relevant chromosomes or genes can be analysed according to the clinical question.
The testing process therefore varies from one pregnancy to another. The healthcare or genetics team will consider the reason for testing, available screening results, family history and the specific condition being investigated before determining which test and laboratory analysis are most appropriate.
What Is Chorionic Villus Sampling (CVS)?
Chorionic Villus Sampling (CVS) is an invasive prenatal diagnostic test that examines a small sample of chorionic villus tissue from the placenta. It can be used to investigate certain genetic or chromosomal conditions during pregnancy when diagnostic testing is considered appropriate.
CVS can be used to investigate specific genetic or chromosomal conditions. The laboratory analysis is selected according to the clinical question, family history or other findings during pregnancy.
CVS is usually performed during the first trimester, commonly from around 11 weeks of pregnancy. The exact timing depends on the clinical circumstances and the healthcare team's assessment.
Using ultrasound guidance, a small sample of placental tissue is collected through either the abdomen or, in some circumstances, through the cervix. The sample is then sent to a specialist laboratory for genetic or chromosomal analysis.
CVS may be considered when there is a higher chance of a genetic or chromosomal condition, a known inherited condition in the family, a previous pregnancy affected by a genetic condition, or other clinical findings that warrant diagnostic testing.
CVS is an invasive procedure and carries a small risk of miscarriage. The potential benefits, limitations and risks should be discussed with the relevant healthcare or genetics team before deciding whether the test is appropriate.
Note: CVS is a diagnostic test rather than a screening test. It can provide information about specific genetic or chromosomal conditions, but the scope of testing depends on the laboratory analysis requested and the clinical circumstances.
What Is Amniocentesis?
Amniocentesis is an invasive prenatal diagnostic test used to investigate certain genetic and chromosomal conditions during pregnancy. It involves taking a small sample of amniotic fluid, which contains cells and other substances from the pregnancy, for laboratory analysis.
Amniocentesis is usually performed from around 15 weeks of pregnancy, although the timing can vary depending on the clinical circumstances. The procedure is carried out under ultrasound guidance, with a fine needle passed through the abdomen into the amniotic sac to collect a small amount of fluid.
The sample can be analysed for specific genetic or chromosomal conditions based on the reason for testing. This may include conditions such as Down’s syndrome, Edwards’ syndrome and Patau’s syndrome, as well as certain inherited single-gene conditions when there is a particular reason to investigate them.
Amniocentesis may be offered when prenatal screening indicates a higher chance of a chromosomal condition, an ultrasound scan identifies findings that warrant further investigation, or there is a known genetic condition or variant in the family. The exact analysis depends on the clinical question and information available about the pregnancy.
Because amniocentesis is an invasive procedure, it carries a small risk of complications, including miscarriage. The potential benefits, limitations and risks should be discussed with the relevant healthcare or genetics team so that parents can make an informed decision about whether diagnostic testing is appropriate.
What Happens During Prenatal Genetic Testing?
The prenatal genetic testing process depends on the type of test being performed and the reason for testing. It generally involves reviewing relevant information, collecting an appropriate sample, and analysing it in a specialist laboratory.
Hover over each box for more detailed information!
Every pregnancy is different. The type of prenatal genetic test, sample required, laboratory analysis and timing of results depend on the individual circumstances and the specific genetic or chromosomal condition being investigated.
How Are Prenatal Genetic Test Results Interpreted?
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Lower-Chance Screening Result: A lower-chance result means the screening has estimated a lower likelihood of the condition being assessed, but it does not completely exclude the possibility.
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Higher-Chance Screening Result: A higher-chance result indicates an increased likelihood of a particular condition, but it does not by itself confirm that the baby has the condition.
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Diagnostic Test Result: Results from diagnostic testing are interpreted according to the specific genetic or chromosomal condition and genetic findings examined by the laboratory.
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Uncertain Findings: Some genetic analyses may identify findings whose clinical significance is not immediately clear and may require further specialist interpretation or additional testing.
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Clinical Context: Results are considered alongside the reason for testing, family history, ultrasound findings, previous screening results and other relevant clinical information.
What Happens After a Prenatal Genetic Test Result?
What happens after a prenatal genetic test result depends on the type of test performed, the condition being investigated and the result obtained. Screening and diagnostic results can lead to different next steps, and results are considered alongside the wider clinical circumstances of the pregnancy.
If a prenatal screening test indicates a lower chance of the condition being assessed, no further genetic testing may be needed. However, a lower-chance screening result cannot completely exclude a genetic or chromosomal condition, and routine antenatal care and any recommended follow-up should continue.
If screening indicates a higher chance of a genetic or chromosomal condition, further assessment may be offered. Depending on the circumstances, this may include additional screening, ultrasound examinations or diagnostic testing such as chorionic villus sampling (CVS) or amniocentesis.
When diagnostic testing identifies a genetic or chromosomal condition, the result is reviewed in the context of the specific condition, the findings of the test and other relevant information about the pregnancy. Further discussions may include the implications of the result, available medical support and any additional assessments that may be appropriate.
If testing identifies a genetic variant whose significance is uncertain, further specialist interpretation may be required. In some circumstances, additional testing or review of family genetic information may help clarify the finding, although this is dependent on the particular genetic condition and test performed.
Where a known familial genetic condition has been investigated, the result may also have implications for other family members or future pregnancies. The appropriate next steps depend on the inheritance pattern, the genetic finding identified and the circumstances of the family.
The next step depends on the result and the reason for testing. Prenatal genetic test results should be interpreted by appropriately qualified healthcare or genetics professionals, who can explain what the findings mean and whether any further assessment or follow-up is appropriate.
Prenatal Genetic Testing and Genetic Counselling
Genetic counselling can be an important part of prenatal genetic testing, particularly when there is a known genetic condition in the family, a previous pregnancy affected by a genetic or chromosomal condition, or a prenatal screening result that indicates a higher chance of a particular condition.
Before testing, genetic counselling can help explain the reason for testing, what the proposed test can and cannot identify, and whether the test is a screening or diagnostic test. The discussion may also consider family history, previous genetic test results and the possible implications of different results.
After testing, genetic counselling can help put the result into context. This may include explaining a higher-chance screening result, a diagnostic finding or an uncertain genetic finding, as well as discussing whether further testing or follow-up may be appropriate.
Prenatal genetic testing can also have implications for parents, relatives and future pregnancies when a genetic condition or familial variant is identified. Understanding the inheritance pattern and the significance of the genetic finding can help families consider what the result may mean beyond the current pregnancy.
The decision to have prenatal genetic testing is personal, and counselling should provide balanced information rather than direct parents towards a particular choice. For more information about the counselling process itself, see our Genetic Counselling service.
Genetic counselling supports informed decision-making. It helps families understand the purpose and limitations of prenatal genetic testing, consider the possible implications of different results and discuss appropriate next steps with a qualified genetics professional.
Prenatal Genetic Testing for Families With a Known Genetic Condition
Prenatal genetic testing may be considered when a genetic condition or specific genetic variant is already known within a family. The most appropriate approach depends on the condition, how it is inherited, whether the familial genetic variant has been identified, and the stage of pregnancy.
| Family Circumstance | Example | What May Be Considered |
|---|---|---|
| Known genetic variant | A parent has a confirmed pathogenic or likely pathogenic variant | Targeted prenatal testing may be used to investigate whether the baby has inherited the known familial variant. |
| Known inherited condition | A genetic condition is present in the family and the inheritance pattern is established | Prenatal testing may be considered when there is a clinically significant possibility that the baby could inherit the condition. |
| Previous affected pregnancy or child | A previous pregnancy or child was diagnosed with a genetic condition | If the genetic cause is known, testing in a subsequent pregnancy may be designed to look specifically for the same genetic change. |
| Genetic cause not yet identified | A suspected inherited condition is present in the family but the responsible variant has not been confirmed | Further genetic assessment may be needed before a targeted prenatal test can be considered. |
When a familial genetic condition is already known, prenatal testing can often be tailored to the specific genetic finding. Where the responsible variant has not yet been identified, additional genetic assessment may be required before targeted prenatal testing is possible. The available options depend on the condition, inheritance pattern and individual pregnancy.
Frequently asked questions
In this section you can check frequently asked questions.
What is prenatal genetic testing?
Prenatal genetic testing involves genetic or chromosomal testing during pregnancy to investigate whether an unborn baby may have certain genetic conditions or chromosomal differences. The type of test depends on the condition being investigated and the circumstances of the pregnancy.
What is the difference between prenatal screening and diagnostic testing?
Prenatal screening estimates the chance that a baby may have certain genetic or chromosomal conditions, while diagnostic testing investigates whether a specific condition is present. Screening does not provide a diagnosis.
Is NIPT a diagnostic prenatal genetic test?
No. Non-invasive prenatal testing (NIPT) is a screening test. It can estimate the chance of certain chromosomal conditions, but a higher-chance result generally requires further assessment and, where appropriate, diagnostic testing such as CVS or amniocentesis.
When is prenatal genetic testing recommended?
It may be considered when there is a known genetic condition or variant in the family, a previous pregnancy affected by a genetic or chromosomal condition, a higher-chance screening result, or ultrasound findings that warrant further investigation.
What conditions can prenatal genetic testing detect?
Depending on the test, prenatal genetic testing may investigate chromosomal conditions such as Down’s syndrome, Edwards’ syndrome and Patau’s syndrome, as well as certain inherited single-gene conditions. The range of conditions depends on the specific genetic analysis.
Can prenatal testing be performed if a genetic condition runs in my family?
Yes. If a genetic condition is known in the family, prenatal testing may be considered, particularly when the genetic variant responsible for the condition has already been identified. Targeted testing may then be possible.
What is chorionic villus sampling (CVS)?
CVS is an invasive prenatal diagnostic test in which a small sample of placental tissue is collected for genetic or chromosomal analysis. It is usually performed earlier in pregnancy than amniocentesis.
What is amniocentesis?
Amniocentesis is an invasive prenatal diagnostic test that involves collecting a small sample of amniotic fluid using ultrasound guidance. The sample can then be analysed for specific genetic or chromosomal conditions.
Does prenatal genetic testing carry any risks?
Non-invasive screening tests such as blood tests do not involve the procedural risks associated with invasive testing. CVS and amniocentesis are invasive procedures and carry a small risk of complications, including miscarriage. The potential benefits and risks should be discussed before testing.
What does a higher-chance prenatal screening result mean?
A higher-chance result means that the screening has estimated an increased likelihood of the condition being assessed. It does not confirm that the baby has the condition. Further screening or diagnostic testing may be offered depending on the circumstances.
What happens if a diagnostic test identifies a genetic condition?
The result is reviewed in the context of the specific condition, the genetic findings and other information about the pregnancy. The healthcare or genetics team can explain the result and discuss any appropriate further assessment, support or follow-up.
Can prenatal genetic testing give an uncertain result?
Some genetic tests can identify findings whose clinical significance is not immediately clear. An uncertain finding may require specialist interpretation and, depending on the circumstances, additional genetic or family testing.
How long do prenatal genetic test results take?
The timeframe varies according to the type of test and laboratory analysis. Some analyses can provide an initial or rapid result within a few days, while more detailed genetic testing may take longer.
Is prenatal genetic testing compulsory?
No. Prenatal screening and diagnostic testing are choices. Parents should be given appropriate information about the purpose, potential benefits, limitations and risks of a test so they can make an informed decision about whether to proceed.
Discuss Prenatal Genetic Testing With a Specialist
Prenatal genetic testing can involve different screening and diagnostic options, and the most appropriate approach depends on the stage of pregnancy, family history, previous test results and the genetic or chromosomal condition being investigated.
How a Specialist Can Help
Understand Your Options: A specialist can explain the available prenatal genetic tests and the differences between screening and diagnostic testing.
Assess Your Individual Circumstances: Your medical history, family history, previous pregnancies and existing genetic information can help determine which testing options may be appropriate.
Interpret Test Results: If testing has already been performed, a specialist can help explain what the findings mean and whether further assessment may be appropriate.
Making an Informed Decision
Prenatal genetic testing is a personal decision. Specialist guidance can help you understand the potential benefits, limitations and risks of different testing options, allowing you to make an informed choice based on your individual circumstances.
If you are considering prenatal genetic testing or have questions about an existing result, speak with a specialist at London Genetics to discuss your circumstances and the testing options that may be appropriate for you.
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