Amniocentesis
Introduction
Amniocentesis is a way of finding out how a baby is developing before birth it shows whether the baby has a genetic disorder.
All pregnant women in the UK are offered a blood test for Downs Syndrome and other genetic conditions such as Spina Bifida, if you are expecting twins the blood test does not work so a detailed scan is used instead. You are normally told very early on in pregnancy about amniocentesis, normally at your booking in appointment with the midwife. For a women aged 35, the risk of the procedure causing a miscarriage is about the same as the risk of the baby having Downs Syndrome.
An amniocentesis is not without risk it can cause miscarriage in about 1% of women. Which means that about 1 woman in 100 will miscarry because of a direct result of having had an amniocentesis.
Why should it be done?
Amniocentesis is used to detect serious or potentially serious disorders in an unborn child, allowing the parent's time to except a disability or to decide to terminate the pregnancy.
Amniocentesis can also offer the opportunity for early treatment of a disorder while the baby remains in the womb
All pregnant women can have an amniocentesis, but there are some groups of women that are offered it as a routine procedure:
- If a pregnant woman is over 35.
- If AFP (Alpha-fetoprotein) levels in the blood are higher or lower than normal, picked up by the triple or double blood test (a higher level suggests neural tube defects such as Spina Bifida, while a lower level suggest a chromosome abnormality such as Downs Syndrome.
- If you have had a child with a disability already
- If there is family history that suggests a higher chance of a disability.
When should it be done?
Amniocentesis is usually done at around 15 weeks of pregnancy, normally between sixteen and twenty weeks. It can be done earlier than 15 weeks, but is avoided if possible because of the higher risk of miscarriage or of club foot in the baby.
Amniocentesis can be used in late pregnancy to look for anaemia in babies with Rhesus factor disease.
Chorionic Villus sampling another intrauterine test or CVS as it is abbreviated to, can be carried out earlier than amniocentesis at about 10 weeks. It carries the same risks of miscarriage but instead of testing the amniotic fluid, tests tissue from around the site of the placenta.
Why is it necessary?
Amniocentesis diagnoses the likelihood of the baby developing one or more conditions, which may be either inherited or develop during the pregnancy. The Double or Triple blood test cannot diagnose a disorder it can only give an indication or a likelihood of the baby developing a disorder. Disorders that may be detected by amniocentesis include:
- chromosomal conditions, like Downs Syndrome,
- disorders of the musculoskeletal system, like Duchenne muscular dystrophy, cystic fibrosis or Spina Bifida,
- blood disorders, like Haemophilia, Thalassaemia or Sickle Cell Anaemia,
- Metabolic disorders, like Phenylketonuria or Antitrypsin Deficiency.
How is it performed?
Some hospitals offer you a local anaesthetic to numb your abdominal (tummy) wall. If you have any concerns about local anaesthetic, always ask a healthcare professional.
Ultrasound scan ensures a needle can be passed safely through the walls of your abdomen and womb into the amniotic fluid in which the baby is floating.
A sample of fluid is be sucked out with a syringe.
The fluid contains cells shed from the babies skin and various substances secreted by the baby, the samples obtained can be used for diagnosis. Every cell from your baby contains a complete set of thier DNA.
Results
Once the sample is obtained it has to be sent away to be cultured (grown).
It takes around one to three weeks for you to get results of the test. Sometimes you can obtain preliminary results earlier. Some hospitals will require you to pay for early results as they are not done on the NHS.
The results of amniocentesis give a very accurate diagnosis for your baby.
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