Amniocentesis and CVS: diagnostic tests explained

If amniocentesis or CVS has been mentioned to you, it’s usually because a screening test (like NIPT or the nuchal translucency scan) has come back with a higher-chance result, or because something in your history means you’ve been offered more certainty. That’s an anxious place to be, so let’s start with the most important thing: being offered one of these tests does not mean something is wrong with your baby. It means a question has been raised, and these tests exist to answer it properly.

Screening asks “what’s the chance?” — diagnosis answers “yes or no”. NIPT and the combined first-trimester screen are screening tests: very good at estimating chance, but never definitive. CVS (chorionic villus sampling) and amniocentesis are diagnostic tests — they look directly at your baby’s chromosomes or genes, so they can tell you with near-certainty whether a condition like Down syndrome, or a specific genetic condition you both carry, is actually present.

CVS is done earlier, usually between about 11 and 14 weeks. Guided by ultrasound, a specialist takes a tiny sample of the placenta (which shares your baby’s genetic makeup), most often with a fine needle through your tummy.

Amniocentesis is done a little later, usually from about 15 weeks. Again with ultrasound guidance, a thin needle takes a small sample of the amniotic fluid around your baby, which contains your baby’s cells. The fluid replaces itself, and your baby isn’t touched.

What it’s like. Both procedures take only a few minutes, and most people describe them as uncomfortable rather than painful — a sharp scratch and a strange pressure or cramping, a bit like a period pain. You’ll be asked to take it easy for the rest of the day, and mild cramping afterwards is common. You can usually bring a support person, and it helps to.

The risk question. This is the part most people worry about, so here it is plainly: both tests carry a small chance of miscarriage. You’ll often hear figures around 1 in 100 to 1 in 200 quoted, and more recent evidence suggests the added risk in experienced hands is smaller than those older numbers. Your specialist will give you their own clinic’s figures — ask them directly; it’s a completely fair question. This small risk is exactly why these tests are never routine: they’re offered only when the information genuinely matters, and it’s always your choice whether to go ahead. Choosing not to test is a valid answer too.

Getting results. A rapid result for the most common chromosomal conditions often comes back within a few days, with the full, detailed result taking around two weeks, depending on the lab and what’s being tested. The waiting is genuinely hard — plan something kind for yourself in that window, and lean on the people who know.

If the result is reassuring, which it very often is, that’s usually the end of it — you carry on with normal pregnancy care, with the question answered for good. If the test confirms a condition, you won’t be left to work out what it means alone: you’ll be supported by your obstetrician and a genetic counsellor, who can explain the condition, connect you with families and support organisations, and talk through your options without judgement, whatever you decide.

Afterwards, call your maternity unit if you have heavy bleeding, fluid leaking, fever, or cramping that gets worse rather than settling — they’ll want to hear from you, and checking is always the right call.

This is one of those crossroads in pregnancy where information helps but a good conversation helps more. Your obstetrician, midwife or a genetic counsellor can go through your specific situation — your results, your numbers, your options — for as long as you need. Don’t be afraid to book that conversation twice.