Amniocentesis | Definition, Risks And What To Expect

Amniocentesis is a type of diagnostic test you might be recommended to have during pregnancy.

This test is used to diagnose whether your baby has any genetic disorders. Your healthcare provider might have concerns about an ultrasound result. Or perhaps your family or personal medical history indicates there’s an increased risk of birth defects.

Here we will explore how an amniocentesis is performed, explain the risks, and give you an idea of what to expect.

What is amniocentesis?

An amniocentesis is a prenatal screening test where a small amount of amniotic fluid is removed from around the developing baby.

A thin, hollow needle is passed through the abdomen and the wall of the uterus, and into the amniotic sac and fluid. This is done without touching the baby or the placenta.

Approximately 15mls of amniotic fluid is taken. This is a very small amount of amniotic fluid compared with the total amount of fluid normally found at 16 weeks (about 150mls).

What does an amniocentesis test for?

An amniocentesis test will determine the amount of a substance called alpha-fetoprotein in the amniotic fluid. A large amount of this protein is linked to babies who have neural tube defects, such as spina bifida.

Another diagnostic test is carried out on fetal cells also present in the amniotic fluid. These chromosomes can be checked for several types of genetic conditions.

Amniocentesis can detect whether the baby is a carrier of Down Syndrome or other less common types of neural tube defect, such as cystic fibrosis, sickle cell disease, or Tay Sachs.

Who is offered an amniocentesis?

Not every pregnant woman will be offered this diagnostic test. Your healthcare provider might recommend a test if you:

• Are aged 37+ years when you will give birth
• Have already had a child with a genetic condition, such as Down Syndrome
• Have shown an increased risk in first-trimester screening test
• Have had abnormal ultrasound findings
• Have (or your partner has) a family history of being a carrier of genetic conditions.

Why are women over the age of 37 offered an amniocentesis?

As the age of the mother increases, the chances of her having a baby with a birth defect also increase. The most common chromosome abnormality to occur in older mothers is Down Syndrome.

In women aged between 37 and 40 years, the chance of having a baby with a birth defect is approximately 1 in 100. Beyond the age of 40 years, this chance increases even further.

The incidence of neural tube defects such as spina bifida doesn’t seem to be correlated with age.

When is amniocentesis performed?

An amniotic fluid test is usually performed between the 15th and 20th weeks of pregnancy. There has been found to be an increased risk of complications if this testing is done before 15 weeks.

It’s not so common, but amniocentesis can be done around 32-39 weeks of pregnancy, to assess the lungs of the baby for fetal lung maturity.

A baby under 32 weeks is unlikely to have reached full lung development.

If you need to birth your baby early, fetal lung maturity is an important way to tell whether your baby’s lungs are mature enough.

Amniocentesis risks

Amniocentesis carries some risks, which need to be weighed up against the information you are hoping to gain from the test.

• Miscarriage. The risk of miscarriage is 1 in 200 (0.5%) tests performed. Recent research shows some evidence the risk of miscarriage is lower than previously thought
• Infection. There’s a very small risk of introducing infection. Mothers who have blood infections such as hepatitis B or C, or HIV, are at risk of transmitting them to their baby
• Leaking of amniotic fluid. In some cases, mothers have been known to leak amniotic fluid through the puncture site after the procedure
• Rh sensitization. Sometimes the procedure causes red blood cells from the mother and the baby to mix. Mothers who have an Rh-negative blood group will need to have an injection of Rh immune globulin after the procedure, to protect the baby’s red blood cells from being attacked.
• Needle injury. On rare occasions, the fetus can move while the procedure is going ahead and catch on the needle. Serious injury is unlikely if this were to happen.

Amniocentesis procedure

Before having an amniocentesis test, your health care provider will ensure you have all the information you need, so you feel prepared.

Before the procedure, your bladder should be full, as this helps lift the uterus up out of the pelvis, making it easier to see the baby via ultrasound.

The ultrasound scan is performed before the amniocentesis, so the pregnancy and the placenta can be checked, and also to see if there’s one or multiple babies.

The scan is quite painless and involves spreading warm gel over the lower abdomen and then moving an ultrasound probe gently over the skin surface.

Your belly area is then cleaned and a thin needle is inserted into the uterus. The needle enters the amniotic sac and is watched all the time on the ultrasound screen.

During this time you’ll need to lie very still. Your health care provider will reassure you and remind you to stay relaxed and still.

A small sample of amniotic fluid is then removed through the needle. It takes only about one minute to draw up 15-20mls of fluid into a syringe. After the needle is removed and the baby is checked again, you’ll be able to get dressed and go home.

It’s recommended that you take things quietly for the rest of the day. The next day, you can return to your usual routine, although your health care provider might recommend you avoid strenuous activity or sex for a few days more.

Is an amniocentesis painful?

Everyone’s experience with the procedure can be different.

Almost without exception, women experience far less discomfort from the test than they had expected. Any discomfort is usually similar to what you might feel during a simple blood test.

One anonymous mother describes her experience:

“The doctor talked through everything he was doing, the needle going in felt less than plucking a hair, drawing out the liquid I felt a tiny sensation”.

Lucinda recalls her experience:

“I found the procedure extremely stressful but quite painless. My husband held my hand and watched the whole time, I kept my eyes closed. I experienced mild cramping for a few hours afterward but no bleeding”.

Sammie’s experience, however, was quite different from the other mothers’:

” They could not get a good angle to get the placenta because of its position, and the position of the baby, so they had to go in from 3 different angles. I was in such agony, the pain was unbearable”.

Amniocentesis side effects

It’s very rare to have any problems or side effects after the test.

Occasionally there can be some mild discomfort due to a little bruising under the skin. Some women also experience some uterine cramping, which usually goes away.

Contact your health care provider immediately if you experience:

• Any vaginal bleeding
• Leaking of fluid from the vagina
• Any sign of infection, such as pain or fever.

Amniocentesis test results

The results of the spina bifida test are usually available after one week. Usually, a diagnosis of spina bifida is made more accurately with an ultrasound examination.

The chromosome test results for Down Syndrome take from 10 to 21 days. This is because the cells are required to grow in a laboratory for a certain number of days.

You will receive the amniocentesis results from the pregnancy health care provider looking after you and your pregnancy.

Amniocentesis accuracy

The results of amniocentesis are usually very accurate – 99.9% accurate in detecting genetic disorders, and over 90% accurate in diagnosing neural tube defects. Even a thorough 18-20 week ultrasound test, however, doesn’t look for every condition.

What amniocentesis cannot detect

Amniocentesis doesn’t detect structural problems such as cleft palate, club foot, or heart defects. These things are usually picked up by the ultrasonographer during the scan.

“The results of the amnio came back showing no problems; however the doctors advised us that meant our daughter didn’t have any of the conditions they can test for with an amnio. 5 weeks after she was born she was diagnosed with a rare disability that could not be tested for using an amniocentesis.

“I guess I just want to educate women out there that an amnio cannot rule everything out. Looking back, I wish I had known that, as I doubt I would have had one” – Lucinda

Chorionic villus sampling vs amniocentesis

Chorionic villus sampling (CVS) is when a small number of cells are removed from the placental tissue with the use of a needle. This differs from amniocentesis, as placental tissue is removed instead of the amniotic fluid around the baby.

Similar to amniocentesis, CVS is done mostly through the abdomen (transabdominal). However, it can also be done through the vagina and cervix (transcervical).

Like amniocentesis, CVS is recommended for mothers who have a higher risk of genetic disorders or inherited diseases through family history, blood tests, or ultrasound.

CVS is performed earlier in the pregnancy, around week 10-13. This means you can receive results earlier and make decisions sooner.

Amniocentesis is usually done later in the second trimester (15-20 weeks); research has shown it’s generally safer than CVS screening at this stage of pregnancy.

Transabdominal CVS screening is shown to be safer and the preferred method of testing for birth defects if early screening is required.

How long after amniocentesis does miscarriage occur?

In most cases, if you’re going to miscarry after an amniocentesis, it would happen in the first few days. But it has been known to occur within around two weeks.

Miscarriage could be caused by any of the risk factors and not just as a result of the procedure.

There’s no evidence to suggest there is anything that can prevent it from happening, but it’s good to rest for a few days and avoid strenuous activity.

Is amniocentesis worth the risk?

The decision to have an amniocentesis is a very personal one. You’ll have to weigh the risks of having the test against the benefits of knowing whether something is wrong with your baby.

Only you and your partner will be able to make that decision, based on all the information given to you.

Your healthcare provider can provide all the information you need to help guide your choice.

Linda had an amniocentesis and says…

“I was offered an amnio at age 29 due to having antibodies similar to rhesus factor; although mine are anti-I and anti-c, both attack the unborn baby in the same way as the rhesus does. My husband was positive for both the antigens and so we were offered the amnio to check the DNA make-up of our son so they could determine his blood typing.

“The procedure itself hurt quite a bit for me, but the emotional pain was worse. As it turned out, our son ended up being positive for both antigens so I ended up back at the same hospital every 3 weeks for the remainder of my pregnancy to monitor him through special ultrasound.

“I would do it again if necessary, just so I knew what to expect from the pregnancy. That may seem selfish, but an antibody (or ISO) pregnancy is extremely stressful, and to avoid that stress would be worth it”.