Genetic testing in pregnancy: NIPT, CVS and amniocentesis explained
During pregnancy, you will be offered a series of tests to check how your baby is developing. Some of those tests look at the likelihood that your baby has a chromosomal condition, such as Down's syndrome, Edwards' syndrome or Patau's syndrome. Understanding the difference between the types of test available, what each one can and cannot tell you, and what happens next is important for making decisions that feel right for you. This article explains the main options: the combined screening test, non-invasive prenatal testing (NIPT), chorionic villus sampling (CVS) and amniocentesis. It is based on NHS guidance.
Screening versus diagnostic testing: an important distinction
The single most useful thing to understand about prenatal testing is the difference between a screening test and a diagnostic test. The terms sound similar but they mean very different things, and mixing them up can lead to a great deal of unnecessary anxiety or, equally, unwarranted reassurance.
A screening test does not give a definitive answer. Instead, it gives a probability, a numerical estimate of the chance that your baby has a particular condition. A result described as "higher chance" or "lower chance" is not a diagnosis. It is a way of identifying which pregnancies should be offered further investigation. Many people receive a higher-chance screening result and go on to have a baby who does not have the condition in question. Conversely, a lower-chance result does not rule out a condition entirely, it just means it is less likely.
A diagnostic test, by contrast, gives a definitive yes or no answer. CVS and amniocentesis are diagnostic tests. If the result is clear, you can know for certain whether your baby has the specific chromosomal condition that was tested for. The trade-off is that both procedures carry a small risk of miscarriage. This is why diagnostic tests are not offered to everyone as a first step, and why many people go through screening first to help decide whether a diagnostic test is worthwhile in their situation.
The combined screening test at 12 weeks
The most common first step in chromosomal screening in the UK is the combined test, offered between 10 and 14 weeks and typically done at or near the 12-week dating scan. It combines a blood test measuring certain hormones in your blood with a measurement from the ultrasound scan called the nuchal translucency (the fluid at the back of the baby's neck) along with your age and other factors to produce a chance figure for Down's syndrome, Edwards' syndrome and Patau's syndrome.
The result is expressed as a ratio, for example 1 in 1,000 or 1 in 150. The NHS uses a threshold of 1 in 150 or higher as a "higher chance" result, which triggers the offer of further testing. Results below that threshold are described as "lower chance." The combined test is a screening test, so it does not confirm or rule out any condition: it simply refines your individual probability based on the information available at that point in pregnancy.
NIPT: non-invasive prenatal testing
NIPT, sometimes called cell-free DNA testing, is a blood test taken from the mother, usually from 10 weeks of pregnancy onwards. During pregnancy, small fragments of the baby's DNA circulate in the mother's blood. NIPT analyses those fragments to look for signs of chromosomal differences. It is highly accurate for Down's syndrome (trisomy 21), Edwards' syndrome (trisomy 18) and Patau's syndrome (trisomy 13), and it carries no risk of miscarriage because it is a simple blood draw.
In terms of where NIPT sits in the NHS pathway, it is currently offered as a second-line test rather than a first-line one. If your combined screening result comes back as higher chance, you will be offered NIPT as a next step before any decision about diagnostic testing. This approach allows more people to avoid an invasive procedure altogether, because NIPT can provide a much more refined probability and in many cases gives a result confident enough to help you decide whether CVS or amniocentesis is necessary.
It is worth knowing that NIPT is not yet offered routinely to all pregnant women on the NHS from the outset. If you would prefer NIPT as your first test, or if you want testing earlier than the combined test timetable allows, it can be arranged privately from around 10 weeks. The accuracy and what conditions are included in the analysis can vary between providers, so it is worth asking what the test covers if you go the private route.
NIPT is still a screening test, not a diagnostic one. A high-probability result from NIPT does not confirm a diagnosis on its own, and a low-probability result does not guarantee that your baby is unaffected. For a definitive answer, you would still need CVS or amniocentesis.
CVS: chorionic villus sampling
CVS is a diagnostic procedure that gives a definitive chromosomal result. A small sample of tissue is taken from the placenta, which shares the baby's chromosomal make-up. The sample can be taken either through the abdomen with a fine needle or through the cervix, depending on where the placenta is situated and the clinical preference of the team performing the procedure.
CVS is done from 11 to 14 weeks of pregnancy, which makes it the earliest available diagnostic option. Results are usually available within a few days for the most common chromosomal conditions, with a full analysis following within around two to three weeks in most cases. Because it is done relatively early in pregnancy, CVS is often the preferred option for those who want a definitive answer as early as possible.
The main consideration is the associated miscarriage risk. The NHS quotes a risk of approximately 1% for CVS. This figure comes from large-scale data and represents an average: the actual risk in any individual case depends on factors including the experience of the operator, the position of the placenta, and individual clinical circumstances. Your clinical team will discuss the specific risks in your situation before you decide.
Amniocentesis
Amniocentesis is another diagnostic procedure, but it works differently from CVS. Rather than sampling placental tissue, it involves taking a small amount of amniotic fluid from the sac surrounding the baby. The fluid contains cells shed by the baby, and those cells carry the baby's chromosomal information. A fine needle is passed through the abdomen under continuous ultrasound guidance to draw off the sample.
Amniocentesis is done from 15 weeks of pregnancy, which means it is available later in pregnancy than CVS. Results follow a similar timeline to CVS, with preliminary results for common chromosomal conditions typically available within a few days. The NHS quotes a miscarriage risk of approximately 0.5% to 1% for amniocentesis, making it slightly lower on average than CVS, though again the actual risk depends on individual factors and the experience of the team.
Both CVS and amniocentesis are offered through the NHS if your combined test or NIPT result indicates a higher chance of a chromosomal condition, or if there is a known family history of a genetic condition that means you are at increased risk regardless of screening results. They may also be considered if an anomaly is detected on an ultrasound scan that raises concern about a chromosomal cause.
Making the decision
Deciding whether to have genetic testing, and if so which type, is genuinely personal. There is no universally right answer. Some people want certainty regardless of the risk that comes with obtaining it. Others feel that the miscarriage risk associated with diagnostic testing outweighs the benefit of a definitive answer, particularly if they would not change the course of the pregnancy whatever the result. Some people prefer to avoid all testing. All of these positions are valid.
What tends to help most people is taking time, gathering accurate information, and thinking carefully about what they would actually do with each possible result. If you would find a definitive diagnosis helpful in order to prepare, access support, or make decisions about the pregnancy, diagnostic testing may feel important. If you feel that the probability from screening is enough information for you, or if you would not act on a diagnostic result, screening alone may be sufficient.
Genetic counselling is available through the NHS for people who want help thinking through these decisions. A genetic counsellor can explain your specific results and risk figures clearly, answer questions, and provide a space to talk through the options without pressure in any direction. Many people find this extremely helpful, particularly when they feel pulled between different considerations. Your midwife can refer you.
Frequently asked questions
What is the difference between a screening test and a diagnostic test?
A screening test gives a probability, a statistical likelihood that the baby has a particular condition. It does not provide a definitive yes or no answer. A diagnostic test, such as CVS or amniocentesis, analyses fetal genetic material directly and can confirm or rule out a chromosomal condition with certainty. Diagnostic tests carry a small risk of miscarriage; screening tests do not.
Is NIPT available on the NHS?
Yes, but currently as a second-stage test. On the NHS, NIPT is offered to those who receive a higher-chance result from the combined first trimester screening. It is not routinely offered to everyone as a first-line test. If you would like NIPT earlier or without needing a higher-chance screening result first, it can be arranged privately from around ten weeks of pregnancy.
What is the miscarriage risk from CVS or amniocentesis?
The NHS quotes a miscarriage risk of approximately 1% for CVS and approximately 0.5 to 1% for amniocentesis. The risk may be lower when the procedure is performed by a very experienced operator at a specialist centre. Your doctor will discuss the specific figures relevant to the unit where the procedure would take place.
Do I have to have genetic testing in pregnancy?
No. All genetic testing in pregnancy is optional. You are not required to have screening or diagnostic tests, and choosing not to have them is a valid decision. Many people find it helpful to speak with a genetic counsellor before deciding, particularly if they are weighing up what they would do with a positive result. Your midwife or GP can arrange a referral.
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