Amniocentesis: how it works, risks and indications

Among prenatal tests, amniocentesis holds a unique place. It is the most reliable diagnostic test for several fetal conditions, since it allows the baby's karyotype and DNA to be analyzed directly from cells present in the amniotic fluid. Its invasive nature does, however, raise questions for expectant parents.

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What does amniocentesis involve?

Amniocentesis consists of taking a small volume of amniotic fluid (15 to 30 mL) with a fine needle inserted through the mother's abdomen into the amniotic cavity. This clear fluid, which protects the baby and contributes to the development of the lungs, also contains fetal cells from the baby's skin, digestive tract or urinary tract. These cells make very precise laboratory analyses possible. Because this test confirms or rules out with certainty a suspected chromosomal or genetic abnormality, we call it a diagnostic test rather than a screening test.

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How the procedure goes

Amniocentesis is done in a medical setting (a specialized clinic or a hospital) by an experienced doctor, usually an obstetrician working in fetal medicine. It is performed around 16 weeks, when the volume of fluid is sufficient and the fetus is big enough to avoid any contact.

The procedure starts with an ultrasound to locate the baby, the placenta and the cord. Under real-time guidance, the disinfected skin is punctured with a long, very fine needle. Within a few minutes, the needle passes through the uterine wall and the amniotic sac, and 15 to 30 mL of fluid is gently drawn off. The doctor always chooses a safe path, well away from the baby, and often uses a very fine catheter left in place during the aspiration so that the needle does not move.

You may feel pressure or a cramp, but many patients compare the discomfort to a blood test. The fluid that is removed is quickly replaced by the body. Once the catheter is taken out, an ultrasound confirms that the baby's heart rate is normal and that there is no bleeding.

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What does the mother feel?

The apprehension is often greater than the actual pain. Because the procedure is quick (a few minutes), a general anaesthetic is not needed. A small local anaesthetic is sometimes given at the puncture site to numb the skin, but many doctors feel the anaesthetic injection is just as unpleasant as the amniocentesis itself. You will be lying down and asked to stay relaxed. It is normal to feel stressed, but keep in mind that it does not last long. Most women describe either an almost painless prick or a passing cramp.

Afterwards, your abdomen may feel a little tender or you may have some pulling sensations, but usually nothing intense. It is recommended that you rest for 24 to 48 hours after the procedure, avoid lifting heavy loads and avoid intense activity for two days. This is to minimize the risk of contractions or fluid leaking after the procedure.
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The risks of amniocentesis

Although it is well mastered, amniocentesis does carry certain risks. The main one is miscarriage related to the procedure, estimated at around 0.5 to 1% in singleton pregnancies (slightly more with twins), or roughly 1 case in 200. Other complications, also rare, can occur: uterine infection (less than 0.1%), a slight loss of amniotic fluid or a small amount of vaginal bleeding. Most of these resolve on their own, for example when the membranes seal back up.

After the procedure, we watch for symptoms such as fever, bleeding, significant fluid loss or regular contractions, which call for urgent medical attention.

Worth noting: in the vast majority of cases the procedure goes ahead without consequence, and recent studies even suggest that the real risk of miscarriage may be closer to 0.3% in very experienced centres.
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When is amniocentesis offered?

Given the small risk it carries, amniocentesis is not offered to every pregnant woman, but in specific situations:

• High risk on prenatal screening (e.g., an NIPT positive for trisomy 21, a markedly increased nuchal translucency). Amniocentesis can then confirm or rule out the abnormality.
• An abnormality found on the anatomy ultrasound suggesting a genetic condition (e.g., a heart malformation linked to trisomy 18, echogenic bowel that can point to cystic fibrosis).
• Parents who carry a balanced chromosomal rearrangement or a known genetic mutation, with a risk of passing it on (e.g., a couple who are healthy carriers of cystic fibrosis).
• Maternal age 40 or over, because of the increased risk of trisomy — although today an NIPT is often done first, with amniocentesis only if that result is positive.
• Suspected congenital infection (toxoplasmosis, cytomegalovirus, etc.), in order to look for the organism directly in the amniotic fluid.

Outside of these indications, amniocentesis done purely for reassurance is generally not performed, given the risk involved.

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What can the analysis reveal?

The amniotic fluid collected is sent to a cytogenetics laboratory. Two types of analysis are generally done:

• A full karyotype: this looks at the number and appearance of the 46 chromosomes, making it possible to detect the trisomies (21, 18, 13) as well as other, rarer abnormalities. Results take about 10 to 14 days.
• Targeted genetic tests: when a specific condition is being looked for, a DNA test (e.g., PCR) can identify a specific mutation. If an infection is suspected (such as CMV), the organism's DNA is looked for directly. These results come back within a few days.
• Chromosomal microarray analysis (CMA) is also used more and more; this DNA chip analysis detects microdeletions or microduplications that are invisible on a karyotype. The choice of test depends on the clinical context.‍
• After the amniocentesis, it is recommended that you rest for 24 to 48 hours and avoid strenuous effort. Watch for fever, bleeding, fluid loss or regular contractions: these signs call for prompt medical attention. In the vast majority of cases everything goes normally and the pregnancy continues without complication. Waiting for results can be stressful: talking to people close to you or to professionals can help. A reassuring result often brings great peace of mind. If an abnormality is confirmed, the medical team will be there to discuss the options with you and support you in your decisions.

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Ultimately, amniocentesis is a valuable test offering unmatched diagnostic accuracy for assessing a baby's chromosomal health. Although invasive, it is now performed in a targeted way, with a great many precautions, and its risk remains very low. As Clinique Prenato points out, "this precise diagnostic procedure, essential in certain situations, fits within an overall approach" to pregnancy follow-up.

If amniocentesis is suggested to you, don't hesitate to ask all your questions: being well informed helps you approach the procedure with confidence and appreciate the value of a tool that can offer great peace of mind. A few minutes of discomfort can sometimes shed light on the whole rest of the pregnancy. And above all, the final decision is yours: choose what will let you and your partner welcome your baby as serenely as possible.