Amniocentesis, CVS and fetal procedures

What CVS and amniocentesis are

In CVS, a fine needle passed through the abdomen under ultrasound collects a tiny sample of the developing placenta, called chorionic villi. These cells almost always carry the same genetic make-up as the baby. In amniocentesis, a similar needle draws about 15–20 ml of the fluid surrounding the baby, which contains cells shed by the baby’s skin and urinary tract.

Both samples can be tested for chromosomal conditions such as Down syndrome, Edwards syndrome and Patau syndrome, for specific single-gene conditions such as thalassaemia when the family mutation is known, and, with amniocentesis, for certain infections.

A diagnostic test answers only the questions it is set up to ask. A normal chromosome result is very reassuring for the conditions tested, but it does not rule out every possible problem, such as some heart conditions or developmental conditions that are not chromosomal. This is why the anomaly scan remains important even after a normal result.

Why a diagnostic test might be offered

• A higher-chance result on combined first-trimester screening or NIPT
• An increased nuchal translucency, usually 3.5 mm or more
• A structural finding on the NT, anomaly or echo scan
• Both partners carrying thalassaemia trait or another recessive condition, such as sickle cell
• A known genetic condition in either parent, or a previous baby with a chromosomal or genetic condition
• A parent carrying a balanced chromosomal rearrangement
• Suspected infection in the baby, such as CMV or toxoplasmosis, based on scan findings and maternal blood tests

CVS or amniocentesis: how they compare

The main difference is timing. CVS gives an answer several weeks earlier, which some couples value; amniocentesis is preferred for some indications and later findings.

The position of the placenta, the baby’s position and the week of pregnancy at which the concern arises often decide which procedure is practical. For example, a high-chance result from the NT scan at 12 weeks is usually followed by CVS, whereas a finding on the anomaly scan at 20 weeks is followed by amniocentesis.

Laboratory tests on the sample

The test chosen depends on the question being asked. Results are discussed with you at each stage, and the reasons for choosing a particular test are explained during counselling.

• QF-PCR: rapid test for Down, Edwards and Patau syndromes, and sex chromosome numbers, in about 2–3 days (sex is not disclosed)
• Karyotype: examines all chromosomes under the microscope; picks up larger rearrangements
• Chromosomal microarray: detects small missing or extra pieces of chromosomes; often recommended when a structural finding is present
• Targeted gene testing: for a known family mutation, such as beta-thalassaemia or spinal muscular atrophy
• Exome sequencing: considered in selected situations after genetic counselling, when other tests are normal
• Infection PCR: on amniotic fluid, for CMV, toxoplasmosis and other infections

Before the procedure

Before deciding, it helps to think about what you would do with each possible result. Some couples want certainty in order to prepare for a baby with special needs, others would consider ending the pregnancy for a serious condition, and some prefer not to test at all. There is no wrong answer, and you will not be pressed either way.

A counselling visit comes first. It covers the reason for testing, what each laboratory test can and cannot detect, how long results take, the risks of the procedure and what options exist if a condition is confirmed. Many couples decide against testing after counselling, and that decision is respected.

Your blood group, and HIV, hepatitis B and hepatitis C status, are checked beforehand, because these can affect the procedure. If you carry thalassaemia trait, your partner’s report and any previous genetic reports should be brought so that the laboratory can test for the right mutation. No fasting is needed, and you can eat normally.

What happens on the day

A detailed scan first checks the baby, the placenta and the fluid, and maps a safe needle path away from the baby and blood vessels. The skin is cleaned with antiseptic, and a local anaesthetic may be used. Under continuous ultrasound guidance, the fine needle is passed through the abdomen and the sample is collected in a minute or two.

Most women describe a brief pressure or period-like cramp. The baby’s heartbeat is checked afterwards, and you rest for a short while before going home. It is a good idea to have someone accompany you and to plan a quiet rest of the day rather than returning to work. In twin pregnancies, each baby is sampled separately and the procedure is planned with extra care to identify which sample belongs to which baby.

Risks and how they are reduced

Current evidence suggests that the additional miscarriage risk from CVS or amniocentesis, when performed by trained operators, is about 0.5% or less, and possibly much lower. Most miscarriages that do occur after a procedure happen within the first few days.

• Continuous ultrasound guidance and a single, well-planned needle pass where possible
• Leakage of fluid after amniocentesis in about 1–2%, which usually stops on its own
• Serious infection is rare, under 1 in 1,000, and is reduced by careful antiseptic technique
• Anti-D injection within 72 hours for Rh negative mothers, to prevent sensitisation
• Occasionally the sample is insufficient or the culture fails, and the procedure needs repeating; this happens in roughly 1 in 100 procedures
• Women with HIV or hepatitis B and a high viral load are counselled carefully, as testing can carry a small risk of passing infection to the baby
• Very rarely, the result reflects the mother’s cells rather than the baby’s; laboratories check for this

Most women who have CVS or amniocentesis go on to have an uncomplicated pregnancy.

Aftercare and receiving results

Take it easy for the rest of the day and avoid heavy lifting or strenuous exercise for a day or two. Mild cramping and a little spotting can occur; paracetamol is safe. Sexual intercourse can usually resume after a few days if there is no bleeding or leakage.

The rapid QF-PCR result usually comes within 2–3 days and is conveyed to you directly. Full karyotype, microarray or gene results take about 2–3 weeks. If a condition is confirmed, a further consultation is arranged to explain what it means, the outlook for the baby and the options, including continuing the pregnancy with a plan for birth and paediatric care. For future pregnancies, the result may also guide early testing or pre-implantation genetic testing through IVF.

A note on the PCPNDT Act

Pre-natal sex determination is prohibited in India under the PCPNDT Act. The sex of the baby is not detected or disclosed at any scan.

Your care, step by step

1. CounsellingThe reason for testing, the choice of laboratory test, the risks and your options are explained in Gujarati, Hindi or English before you decide.
2. Pre-procedure checksBlood group, infection status and any family genetic reports are reviewed.
3. Ultrasound mappingThe baby, placenta and a safe needle path are located on scan.
4. SamplingA fine needle collects the sample under continuous ultrasound guidance, followed by a heartbeat check and a short rest.
5. Results and follow-upRapid results usually within days and full results in 2–3 weeks, discussed with you in person.

Myths and facts

Frequently asked questions