An amniocentesis is an optional prenatal test to check on the health and development of your baby, particularly to detect chromosomal and genetic abnormalities such as Downs Syndrome and spina bifida. Usually performed between fifteen and twenty weeks, the procedure involves the use of a needle to remove a small sample of the amniotic fluid from the sac surrounding your baby for testing. Amniocentesis is not routinely offered to all pregnant women as it carries a small risk of miscarriage, although you are unlikely to be refused an amniocentesis if you request one.
When might I be offered an amniocentesis?
The test is usually offered if a woman is considered to be at high risk of developing a fetus with chromosomal or genetic abnormalities. This may be because:
- A previous pregnancy was affected by a chromosomal or genetic abnormality
- Your baby is at risk from an inherited disorder such as cystic fibrosis or Huntingdon's disease
- A woman is over 35 years of age and/or earlier screening blood tests have revealed a significant risk of Down's Syndrome for the developing fetus
- An ultrasound has shown physical indications of a chromosomal or genetic abnormality with the developing fetus
An amniocentesis may also be offered in other medical circumstances not relating to the genetic and chromosomal health of the baby, in which case the test may be carried out in later pregnancy. These include:
- Excess amniotic fluid - If your baby is surrounded by too much amniotic fluid an amniocentesis may be used to remove some of the fluid
- Blood incompatibility - If you are rhesus negative an amniocentesis may be used to test whether the baby is rhesus positive
- Early delivery - If early delivery is necessary or you appear to be in premature labour, an amniocentesis may be used to check whether the baby's lungs are sufficiently developed for birth
- Suspected uterine infection - If there is cause to believe you may have developed a uterine infection an amniocentesis can confirm or rule out the diagnosis
What can the amniocentesis tell me?
Unlike other screening tests that can only indicate the probability of risk, an amniocentesis can be highly accurate when it comes to detecting a wide range of chromosomal and genetic abnormalities. In addition, the test can tell you you baby's gender, which is important in detecting disorders transmitted by gender. Among the many abnormalities that an amniocentesis can detect are Down's Syndrome, Spina-bifida, Cystic Fibrosis, Anencephaly, Myotonic Dystrophy, Huntington’s Disease (HD) and Sickle Cell Disease. There are, however, some abnormalities which amniocentesis does not detect, such as hare lip, cleft palate and limb abnormalities.
Do I have to have an amniocentesis if it is suggested?
No. If there is a reason to believe that your baby is at a higher than usual risk of developing an abnormality then you will usually be offered the test, but whether you choose to take it is entirely up to you. Some couples may want the test if they would consider aborting a fetus diagnosed with an abnormality or genetic disorder. Couples who would not consider the possibility of an abortion may still want the test in order to prepare themselves for after the birth should the tests diagnose a problem, or to give them peace of mind for the rest of the pregnancy it the results are clear. On the other hand you may think that the slight risk of a miscarriage that the procedure entails is not worth taking (see below), whatever the results may be.
One of the drawbacks of the amniocentesis is that it is usually conducted in the second trimester - up to week 20 - and the results may not be through for another two or three weeks after that, potentially presenting parents-to-be with very difficult decisions about continuing the pregnancy at a relatively late stage.
What does the procedure entail?
An amiocentesis doesn't usually require a stay in hospital, you will be treated as an outpatient. Before the procedure you will usually be asked to drink plenty of water so that the necessary ultrasound picture will be as clear as possible. You may be offered a choice between a local or a topical anaesthetic, the local anaesthetic involving an extra needle, which you might want to avoid.
Your abdomen is washed with a sterile solution before a thin, hollow needle is inserted into the abdomen under the guidance of ultrasound. The use of ultrasound enables the doctor to find an area of the amniotic sac that is well away from baby, placenta and umbilical cord so that none of these are punctured by the needle. Finding a safe area may take some time, but once located it only takes a few minutes for the doctor to insert the needle and remove the small amount of fluid, about two tablespoons, needed for the tests.
Once the needle is removed you might have some stomach cramps, if so then these are usually quite mild, though some women may experience stronger cramps. You and the baby will be monitored for a while to make sure that the baby hasn't become distressed during the procedure before you can go home, but you will be advised to rest until any cramping has subsided.
The amniotic fluid removed contains cells shed from the baby, all of which contain a complete picture of the baby's DNA: It is these cells that are analysed in the lab tests.
What are the risks?
Amniocentesis does carry a small risk of spontaneous miscarriage, somewhere between 0.5 and 1 per cent - the NHS gives the miscarriage risk as 1%. However, this risk is doubled for multiple pregnancies, such as twins. There is also the risk that the mother may develop an infection (also around 0.5%), high temperature, uterine contractions, soreness in the abdominal area and a leakage of blood or amniotic fluid which will require a period of bed rest. You should see your doctor if any of these occur.
When do we get the results?
Amniocentesis involves detailed analysis and time is required to grow enough of your baby's cells for the tests to be carried out, so there will be a wait for your results: The results are usually ready between ten days and three weeks later. Occasionally you will be able to receive preliminary results more quickly. If it is very important to you that your receive results quickly then you might want to consider looking into private options for testing - your doctor should be able to provide you with details.