Genetic Screening in Pregnancy: NIPT, the Combined Test, Amnio and CVS

TL;DR: Prenatal genetic tests fall into two groups: screening tests, which estimate the chance of a condition, and diagnostic tests, which confirm it. The combined test and NIPT are screens; amniocentesis and CVS are diagnostic. Whether to have any of them is a personal, optional choice, and no result obliges you to do anything. This is education, not medical advice, and a clinician or genetic counsellor can talk the options through with you.

At some point in pregnancy you may be offered tests that look at your baby's chance of certain genetic conditions. It can feel like a lot to take in, especially when the words sound similar and the leaflets pile up. This piece is here to slow things down and explain what the tests are, what they look for, and how they differ from one another. It is not here to tell you what to choose. That part is entirely yours.

Screening versus diagnostic: the core difference

The single most useful thing to hold onto is that there are two kinds of test, and they answer different questions. A screening test estimates the chance that your baby has a condition. It gives you a number, a likelihood, not a yes or no. A diagnostic test goes further and confirms whether a condition is actually present. Screening tells you how likely; diagnosis tells you whether. Understanding that distinction makes everything else easier to follow, and it is worth raising early, alongside the questions worth asking at antenatal appointments.

What the tests look for

In the NHS, the main screening offered in pregnancy looks at three conditions: Down's syndrome (trisomy 21), Edwards' syndrome (trisomy 18) and Patau's syndrome (trisomy 13) [1]. These are chromosomal differences, and the screening estimates the chance that your baby has one of them. The programme that oversees this in England is the NHS Fetal Anomaly Screening Programme [7]. Being offered screening is routine; accepting it is a choice, and some people decide it is not information they want. That is a valid decision too.

The combined test and NIPT (the screens)

The combined test is usually offered between weeks 10 and 14. It brings together a blood test and a measurement taken during an ultrasound scan, along with factors such as your age, to work out a chance figure [1]. If you understand how scans fit into the wider picture, our overview of prenatal tests and scans sets the scene.

NIPT, or non-invasive prenatal testing, is a newer blood test that analyses fragments of the baby's DNA circulating in your blood. It is a very accurate screening test, more so than the combined test, but it is still a screen: it estimates chance rather than confirming a result, and a higher-chance NIPT result would usually lead to a conversation about diagnostic testing [1]. On the NHS, NIPT may be offered after a higher-chance combined test result. Because it needs only a blood sample, it carries no risk to the pregnancy.

Amniocentesis and CVS (the diagnostic tests)

If screening suggests a higher chance, or if there is another reason, you may be offered a diagnostic test that can confirm whether a condition is present.

Amniocentesis involves taking a small sample of the fluid around the baby, usually from around week 15, using a fine needle guided by ultrasound [2]. Chorionic villus sampling (CVS) takes a tiny sample of cells from the placenta, usually earlier in pregnancy [4]. Both give a diagnostic answer rather than a chance figure. The Royal College of Obstetricians and Gynaecologists has a clear patient guide comparing the two [6].

Because both involve taking a sample from inside the womb, they carry a small risk of miscarriage. For amniocentesis, this is quoted as up to about 1 in 200 [3]. For CVS, the risk is less than 1 in 200, rising to around 1 in 100 in a multiple pregnancy [5]. These are the figures the NHS publishes, and a clinician can explain what they mean for your particular situation.

Making sense of it for your own circumstances

Genetic screening can carry an extra weight for African women, who face documented disparities in maternity care, and feeling properly heard in these conversations matters. Our piece on maternal health risks for African women covers that ground, and knowing which specialists do what can help you find the right person to ask. A genetic counsellor is trained specifically to talk through what results might mean without steering you toward any decision.

There is no single right answer here. Some people want every piece of information available; others prefer not to test, or to screen but not go on to a diagnostic test; others change their mind as they go. Each of these is a reasonable path. If you are early in your care and building confidence with clinicians, our guide to a first gynaecologist appointment may help, and our podcast episode on proactive pregnancy care explores similar ground.

How Asele can help

Understanding your options is easier when you can ask questions in plain language and keep track of what you have been offered. Within Asele, you can talk things through with Amara, our assistant, at your own pace, and note down the questions you want to bring to your next appointment. Asele does not make decisions for you or replace your care team; it is a calm companion for the space between appointments.

Whatever you decide, remember that being offered a test is not the same as being told to take it, and a result is not an instruction. The choice, at every step, remains yours to make with the people who care for you.

This article is for education and is not medical advice. Please speak with your midwife, doctor or a genetic counsellor about your own pregnancy and options.

References

1. NHS. "Screening for Down's, Edwards' and Patau's syndromes." nhs.uk
2. NHS. "Amniocentesis." nhs.uk
3. NHS. "Amniocentesis: risks." nhs.uk
4. NHS. "Chorionic villus sampling (CVS)." nhs.uk
5. NHS. "CVS: risks." nhs.uk
6. RCOG. "Amniocentesis and chorionic villus sampling." rcog.org.uk
7. GOV.UK. "NHS Fetal Anomaly Screening Programme." gov.uk