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 chromosome change. 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). The results of which may identify a genetic cause for the findings seen in your baby.
To understand why these genetic tests can be helpful, it is useful to understand a little more about your baby’s genes and chromosomes.
What are genes?
Genes are instructions that tell our bodies how to grow and develop. They contain instructions to make proteins. Proteins perform various functions to keep the body healthy. Genes are made of DNA (deoxyribonucleic acid).
What are chromosomes?
Chromosomes are packages of DNA which contain the genes. They are found in nearly every cell of our body. 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.
Sometimes, a chromosome variation randomly occurs in the sex chromosomes. The most common is a condition called Turner syndrome which usually affects girls and is where one of the X chromosomes is missing. This is only an example of one of the conditions caused by changes in sex chromosomes.
You can obtain further information relating to each of these conditions from the support section at the end of this leaflet.
Occasionally, an unexpected finding on ultrasound can be due to a rarer chromosome change. This is where some genetic material may be missing (deletion), moved (translocation) or has been added to a chromosome (gain, or duplication).
Chromosome deletion
A missing piece of genetic material can happen on any chromosome and can be any size. If the genes in the missing piece contain important instructions for the body, a deletion may result in changes in the way the baby’s organs (for example, heart, kidneys or brain) develop. A deletion can also cause developmental delay and/or health problems in a child after birth. This will depend upon how much of the chromosome has been deleted and where the deletion is.
Chromosome translocation
Translocation is where one portion of chromosome has moved to a different chromosome. The effect of these translocations depends upon whether any genetic material has been lost or gained. Not having enough or having too much genetic information may cause a miscarriage, or the birth of a baby with developmental and/or medical problems.
Chromosome duplication
A chromosome duplication (chromosome gain) is where a chromosome has copied part of itself, resulting in a doubling up of genetic information. This may result in developmental delay and/or health problems in a child.
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 a consideration after birth.
- If you decide to have a test, you will be referred 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 can tell you, with a high level of accuracy, if your baby has a chromosomal change involving extra or missing genetic information.
- 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 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?
The genetic test is called a QFPCR (Quantitative Fluorescence Polymerase Chain Reaction). It 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.
This result is typically available within 2-3 working days. With your agreement, this result will be telephoned to you.
You may choose to receive the result at a further appointment with your specialist doctor.
It may take longer to receive your result. 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.
If the QFPCR has not identified a common chromosome change, the sample is tested in more detail to look for more rare genetic changes.
This second result is known as a microarray.
A microarray can detect small changes in DNA known as copy number variants (CNVs) where genes may be missing (deletion), moved (translocation) or added to a chromosome (gain, or duplication).
This result is typically available within 2-3 weeks.
What results might I expect from the tests?
No significant findings. This means that the QFPCR and microarray have not identified a genetic cause to further understand the unexpected findings seen on your scan.
Confirmed genetic diagnosis. This means that the QFPCR or microarray test has found a chromosome change which explains the unexpected findings seen in your scan. This may give you more information about the condition affecting your unborn baby and may also inform you of the chances of the same condition happening again in a future pregnancy.
Unexpected (incidental) finding. This means the QFPCR or microarray test has identified a chromosomal change which does not explain the unexpected finding seen on your ultrasound scan but may have additional implications to your baby’s health and development.
Uncertain result (variant of uncertain significance or VUS). This means that the microarray test has identified a chromosomal change, but we do not currently have enough evidence to understand fully how this may affect your baby. Sometimes, the laboratory suggests further tests, including parental blood samples to check whether the mother or father have the same chromosome change. If a parent has the same change, and is healthy, this can help us to understand your result.
It is important to remember that genetic testing does not always find the answer and prenatal tests do not rule out an underlying genetic cause. There are other mechanisms that cause genetic conditions which are not yet fully understood or are not currently able to be tested for.
What happens next?
If a genetic cause has not been identified, sometimes further, more detailed genetic testing or genomic testing is considered. This includes a test called Prenatal Exome Sequencing. Your doctor will discuss this further with you, if appropriate.
If your baby has a significant chromosome change or a significant unexpected finding on ultrasound scan, 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.
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.
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’.
