Amniocentesis and CVS

Two tests, two windows

Both are diagnostic tests: unlike screening, they tell you whether a condition is present rather than how likely it is. They differ in what is sampled and when.

Chorionic villus sampling. The NHS screening programme handbook describes CVS as "a transabdominal or sometimes transcervical procedure performed under continuous ultrasound guidance. It is usually performed from 11 to 14 weeks of pregnancy but can be done later. A sample of placental tissue is obtained for chromosomal or genetic analysis."

Amniocentesis. The same source: "a procedure performed under continuous ultrasound guidance. It is usually performed from 15 to 20 weeks of pregnancy but can be done later. A sample of amniotic fluid is obtained for chromosomal or genetic analysis."

RCOG Green-top Guideline No. 8 sets harder boundaries at the early end: "Amniocentesis should not be performed prior to 15+0 weeks gestation. CVS should be performed between 11+0 and 13+6 weeks; it should not be performed prior to 10+0 weeks gestation." Its summary adds that CVS "can be performed between 14+0 and 14+6 weeks' gestation" where required, with individualised counselling.

The risk figure, and why you have seen three versions of it

RCOG GTG 8 states: "Women should be informed that the additional risk of miscarriage following amniocentesis or CVS performed by a skilled operator is likely to be below 0.5%."

The NHS screening programme handbook states: "One in 200 women who have a CVS or amniocentesis will miscarry."

These are the same number expressed differently — one in 200 is 0.5% — but the framing differs in a way that matters. RCOG describes an additional risk above the background risk of miscarriage that exists in any pregnancy, and qualifies it with "performed by a skilled operator". Older figures of 1% that still circulate online predate current practice.

For twins the figure is different. GTG 8: "Women with multiple pregnancies should be informed that the additional risk of miscarriage for twin pregnancy following CVS or amniocentesis performed by a skilled operator is around 1%."

What the sample gets tested for

This is the question most worth asking before you consent, because the answer is not automatic. The FASP handbook explains that samples "are sent to the genomic laboratory for QF-PCR testing", and that "QF-PCR testing detects the evidence of T21, T18 and T13". That is a rapid test covering three conditions.

Broader analysis — microarray testing, which can detect much smaller chromosomal changes, or testing for a specific inherited condition in your family — is arranged separately. Ask explicitly what is being requested on your sample, because a normal QF-PCR result answers three questions and not others.

ACOG's patient information makes the same distinction between screening and diagnostic testing, noting that diagnostic tests "can tell you, with as much certainty as possible, whether your fetus actually has an aneuploidy or specific inherited disorders for which you request testing".

How quickly it should happen

The NHS programme sets timing standards. "Following receipt of higher chance combined or quadruple test results, the PND procedure should be made available to women within 3 working days or fewer", and the procedure "should be completed within 3 working days of women receiving their higher chance or 'no result' NIPT screening results".

The handbook also notes that these procedures "should only be performed by specially trained healthcare professionals" and that "women may be required to attend a tertiary-level centre" — you may be travelling to a different hospital from your booking one, which is normal.

The parts nobody mentions in advance

Before the procedure, GTG 8 asks that "when an invasive test is considered, screening results for blood borne viruses, viral load and antigen test results should be reviewed and individualised risk of viral transmission should be discussed". If you have hepatitis B, hepatitis C or HIV, this is a specific conversation about transmission risk and it should happen before the needle.

The procedure itself is done under continuous ultrasound guidance and takes only a few minutes, though the appointment is longer. Most people describe pressure and period-like cramping rather than sharp pain. Afterwards, cramping for a day or two is common; heavy bleeding, fluid loss, fever or severe pain are reasons to contact the unit straight away.

GTG 8 also asks that "women considering amniocentesis or CVS should receive detailed counselling and pregnancy mapping by suitably trained healthcare professionals", and that care "should be organised in accordance with National Screening Committee Standards". Counselling is part of the procedure, not an optional extra.

Choosing between them, and choosing neither

If you are in the CVS window and want an answer as early as possible, CVS gives it weeks earlier. If you are past 14 weeks, amniocentesis is the option. Where a woman is considering CVS in the 14+0 to 14+6 window, GTG 8 asks for "individualised counselling of the merits of CVS versus amniocentesis".

Declining is a legitimate choice. The FASP handbook lists "no further testing" first among the options after a higher-chance result, alongside NIPT and prenatal diagnosis. Some parents want certainty regardless of what they would do with it; others do not want a procedure with any miscarriage risk attached. Both are reasonable, and the counselling exists to help you decide rather than to steer you.

Practical arrangements worth sorting in advance

Bring someone with you. You will not be sedated and you can usually travel home normally, but most units advise avoiding heavy lifting and strenuous activity for a day or two afterwards, and having a driver removes one thing to think about.

Ask before the appointment how results will be given and by whom. There are usually two results at different times: the rapid QF-PCR result covering trisomies 21, 18 and 13 within a few days, and any fuller chromosome or microarray analysis afterwards. Knowing which call is which prevents a very bad few minutes.

Ask whether your blood group is known and whether anti-D is needed. Invasive procedures can allow fetal cells into the maternal circulation, so women who are rhesus D negative are generally offered anti-D prophylaxis after amniocentesis or CVS. If you are rhesus negative, raise it rather than assuming it is automatic.

Finally, ask what happens if the sample fails or the culture does not grow. It is uncommon, but it happens, and knowing in advance that a repeat may be offered makes the possibility less alarming.