Your antenatal genetic test explained
What is a prenatal invasive test?
A prenatal invasive test can tell you, with a high level of accuracy, if your baby has a genetic condition which is known within your family. Screening tests cannot do this. They are called invasive tests as a fine needle is used to obtain a sample from your pregnancy, either from the amniotic fluid around the baby (amniocentesis), or placental tissue (chorionic villus biopsy).
Your screening midwife or genetic counsellor has offered you a prenatal invasive test because you have had a previous baby with a genetic condition, or you or your baby’s father have a family history of a known genetic condition.
To understand why these genetic tests can be helpful, it is useful to understand a little more about your baby’s chromosomes and genes.
What are chromosomes and genes?
Chromosomes are packages of DNA (deoxyribonucleic acid) which contain genes. Genes are the instructions that tell our bodies how to grow and develop.
Chromosomes come in pairs; 23 from mum in the egg and 23 from dad in the sperm. We usually have 46 chromosomes (23 pairs).
Chromosomes are numbered from 1 to 22, and the 23rd pair are called the sex chromosomes (X and Y). The sex chromosomes determine whether we are born a boy (XY) or a girl (XX).
At conception (successful joining of the egg and sperm), the cells begin to divide and multiply and go on to form the baby.
What changes can happen to a chromosome?
Changes to your baby’s chromosomes occur through no fault of your own. Occasionally, this can involve a whole extra copy of a chromosome in some, or all of a baby’s cells. The most common extra copies are of chromosome 21, known as Down’s Syndrome, chromosome 18, known as Edwards’ syndrome or chromosome 13, Patau’s syndrome.
You can obtain further information relating to each of these conditions from the support section at the end of this leaflet.
What is a gene variant?
A variant is a change in the DNA that makes up a gene. This change can be passed on from our parents (an inherited variant) or can occur for the first time in a baby (a de novo variant). Many variants have no impact on health or development, but some can cause a genetic condition.
You have been offered a genetic test because there is a chance your baby may inherit the known genetic change/s already identified in your family.
Choosing to have an invasive test
- Deciding to have a prenatal invasive test is a personal choice. You may wish to consider what the results may mean for your pregnancy and family.
- Members of religious groups may look to their faith traditions for guidance.
- It is your decision whether to have a test. If you decide not to, the care for you and your baby will be tailored for you. Genetic testing may be considered after birth.
- If you decide to have a test, you will be referred by your screening midwife or genetic counsellor to your nearest specialist Fetal Medicine Unit.
What are the risks?
- Prenatal invasive tests are associated with a small additional chance of miscarriage. Nationally this figure is 1 chance in 200 or 0.5%. This chance may be higher depending on your scan findings. There is a chance of less than 1 in 1,000 (0.1%) that an invasive test will cause a serious infection.
- Occasionally, it is not possible to obtain a result and you would be offered a repeat test.
- If you are HIV positive or have the Hepatitis B or C virus, an invasive test may increase the risk of passing this onto your baby. Guidance can be obtained from your specialist doctors.
What are the benefits?
A prenatal invasive test will tell you, with a high level of accuracy, if your baby has inherited the known genetic condition in your family.
- The results may also help you and your health care providers plan your ongoing pregnancy, delivery, and postnatal care.
- The results may help you make decisions around your pregnancy options.
Can I still have a test if I have a twin pregnancy?
Yes. The procedure is the same but technically more difficult. It should be performed in a specialised unit. If genetic testing is being performed on both babies, depending upon the type of multiple pregnancy you are carrying and the position of your babies, it is sometimes necessary to have the needle inserted twice, at a different area of your abdomen (tummy), to ensure both babies’ samples are collected.
The additional chance of miscarriage is quoted as twice as high as in single pregnancies and this relates to your entire pregnancy. Your fetal medicine doctor will explain these risks to you further depending upon your scan findings.
How do I prepare for the test?
To prepare for the test, on the day, you can eat or drink as normal. Wear comfortable clothing and bring your partner or an adult to support you if you wish.
- If you take any blood thinning medications, please discuss this with your healthcare provider prior to your appointment.
- Take it easy for 24 hours after your test and therefore consider any work or childcare arrangements.
Chorionic villus sampling or amniocentesis, which test should I have?
Your midwife and doctor will discuss the most appropriate test for you.
The test you are offered may depend upon your personal choice, how many weeks pregnant that you are, and any relevant scan findings such as the location of your placenta.
Both tests will give you the same results and have the same chance of miscarriage.
What is a chorionic villus sample?
A chorionic villus sample (also known as a CVS) or chorionic villus biopsy (CVB), uses a needle to obtain a small piece of tissue from the placenta (the after birth). It can be performed from 11 weeks into your pregnancy.
One in ten placentas are not in a position that make the test possible, this would mean having to delay your test by a week or two. You may be advised to wait until 15 weeks gestation for an amniocentesis.
What is an amniocentesis?
An amniocentesis uses a needle to obtain a sample of amniotic fluid that surrounds the baby in the womb. It can be performed after 15 weeks of pregnancy.
Preparing to take the test
Your fetal medicine team will discuss your test in detail, confirm associated risks and ask you to sign a consent form.
An ultrasound scan will be performed throughout the test. Sterile (clean) sheets are laid over your tummy and an antiseptic solution is applied to clean the area. If you have a CVS test, a local anaesthetic injection is given to numb the skin, this may briefly feel like a stinging sensation.
Taking the sample
A needle is guided through the wall of the womb into either the placenta or amniotic fluid, away from your baby. Most women feel this as a pushing sensation and tummy cramps, others may feel this as a short, sharp pain. Once the needle is in place, the sample is obtained, the needle is removed, and a small plaster is applied to your tummy. These tests typically take around 5-10 minutes.
After the test
After the test, if you are a Rhesus negative blood group there is a chance of passage of your baby’s red blood cells into your system after an invasive procedure. An injection called Anti D immunoglobulin may be offered to you. This injection is given in the muscle in your arm. The purpose of Anti D is to prevent antibodies developing in case your baby has a different blood group to you, helping to protect a future pregnancy.
You and your partner may be asked to provide a blood sample from your arm to help the laboratory interpret (understand) the results of your baby’s genetic test. You will not receive a result from this blood test unless you have specifically been consented for your own (parental) genetic test.
You can go home soon after the procedure. You may get some period type cramps following the test which should settle with some simple pain relief such as paracetamol. It is fine to bathe or shower following your procedure. Avoid any strenuous activity for 24 hours. If a complication was to occur relating to the test, this would typically be within two weeks, but up to six weeks later. If you experience vaginal bleeding, watery vaginal discharge, worsening of abdominal cramps or a high temperature (38°C or 100.4°F) contact your local maternity unit for advice and assessment.
What genetic tests will be undertaken?
You will be offered a QFPCR (Quantitative Fluorescence Polymerase Chain Reaction). This first genetic test will exclude or diagnose Down’s syndrome, Edwards’ syndrome, Patau’s syndrome and some sex chromosome changes. You may also decide to find out the sex of your baby.
If you choose to accept this test, the results are typically available within 2-3 working days. You may choose to receive this result first or you may prefer to wait until your genetic test result is available and receive the results together.
Choosing to have this additional chromosome test will not delay the result of the genetic test that you have been referred for.
Occasionally it may take longer to receive a QFPCR result. This does not mean there is a problem. For example, if you have had a CVS test, there is a rare condition called confined placental mosaicism (CPM). This can cause different genetic results in the placenta and the baby, and other tests may be offered to investigate this further. Your fetal medicine team will discuss this with you in detail.
The sample is then tested in more detail to look at the gene change/s causing your, or your family’s, known genetic condition. The result of this test is typically available in 1-2 weeks, this may take longer. Your genetic counsellor or screening midwife will arrange a date to contact you with your results.
What results might I expect from the tests?
QFPCR result: no significant findings. If you have chosen to have a QFPCR, this has not identified Down’s syndrome (T21), Edwards’ Syndrome (T18), Patau’s syndrome (T13) and some sex chromosome changes.
QFPCR result: confirmed genetic diagnosis. If you have chosen to have a QFPCR, this may have identified Down’s syndrome (T21), Edwards’ Syndrome (T18), Patau’s syndrome (T13) or a sex chromosome change.
Family gene change/s are not present. This means the genetic test has not identified the known family genetic condition in the baby.
Confirmed genetic diagnosis. This means the genetic test has identified that the baby has the known gene change/s identified in your family.
Confirmed carrier. In some conditions, this means the baby will not be affected by the known genetic condition but would be a carrier in the same way as the parents are.
It is important to remember that these genetic tests will only test for the conditions you have consented to. It does not rule out other genetic or health conditions.
What happens to my sample and storage of data?
As with all health data, the results from your genetic test will be stored securely within the NHS.
What happens next?
If your baby has a significant chromosome or gene change some parents opt to continue with their pregnancy in the knowledge that care for themselves, and their baby will be tailored to individual needs.
Others, after consideration may opt not to continue with their pregnancy in the knowledge that they will be supported in their choices.
Support for you and your family
Please remember, your health care providers are here to support you and will answer any questions that you may have. It can help to speak to your partner, family, friends and sometimes a trained counsellor.
You may need some time to process all of the information and wish to be signposted further to UK websites or charities.
This leaflet is available in Arabic, English, Polish, Punjabi, Romanian and Urdu. It corresponds with the North-East and Yorkshire GMSA video ‘Prenatal invasive testing where there is a family history of a known genetic condition’.
NHS Public Health England (PHE)
Information for pregnant women who are offered a chorionic villus sample (CVS) or amniocentesis diagnostic test
Antenatal Results and Choices (ARC)
Helping parents and professionals through antenatal screening and its consequences
National helpline
Mon-Fri 10am-5.30pm 020 7713 7486
Down’s Syndrome Association (DSA)
Supports people who have Down’s syndrome and their parents or carers from birth into older age
Soft UK
Provides information and support to families relating to Trisomy 13 (Patau’s syndrome) and Trisomy 18 (Edward’s syndrome)
Turner Syndrome Support Society (TSSS)
Provides information on Turner syndrome and the aspects of living with the condition
Genetic Alliance UK
Provides information for patients, families and anyone interested in genetic conditions
This leaflet is available in Arabic, English, Polish, Punjabi, Romanian and Urdu.
It corresponds with the North-East and Yorkshire GMSA video ‘Prenatal invasive testing where there is a family history of a known genetic condition’.
