Amniocentesis
Your family / personal medical history or perhaps an ultrasound result may suggest to your doctor that you have a slightly greater chance of giving birth to an infant with a birth defect. Therefore your doctor may suggest that you undergo an amniocentesis, a test used to detect some foetal abnormalities.
What Is An Amniocentesis?
An amniocentesis is a test in which a small sample of liquor (water) is taken from around the developing baby. A needle is passed through the abdomen, the wall of the uterus (womb) and into the water (liquor) surrounding the baby without touching the baby or the placenta. Approximately 15mls of fluid is taken, this amount being small compared with the total amount of fluid present (at least 150mls) at the time of the test at 16 weeks.
Who Is Offered One?
Women of 37 years and over at the projected time of birth
Women who have already had a child with a problem such as Down’s Syndrome or Spina Bifida
Occasionally amniocentesis is done for other reasons. These include parents who are known to have a chromosomal abnormality or who are known to be at risk of having a baby with one of a number of rare abnormalities of their metabolism.
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 chromosomal abnormality also increases. The most common chromosome abnormality is Downs Syndrome. Between 37 and 40 years of age the chance of having a baby with an abnormality is approximately 1 in 100. Beyond the age of 40 years this chance increases even further. The incidence of Spina Bifida does not seem to be correlated with age.
When Is Amniocentesis Performed?
An amniocentesis is usually performed from 14 weeks gestation.
What Are The Potential Complications?
There is a risk of miscarriage following this test, which is quoted at approximately 1 in 200 tests performed.
What Will Happen On The Day?
It is important that the mother’s bladder is full; this lifts the uterus up out of the pelvis making it easier to see the baby with ultrasound. The ultrasound scan is performed prior to the amniocentesis so that the pregnancy and the placenta can be checked and to determine whether there is a singleton or twin pregnancy. The ultrasound scan is quite painless and merely involves spreading a little warm gel over the lower abdomen and then moving an ultrasound probe gently over the skin surface.
After the ultrasound scan has been performed, the abdomen is cleaned and the needle is inserted into the uterus. The needle is watched all the time on the ultrasound screen. It then only takes about one minute to draw up 15-20mls of fluid into a syringe. After removing the needle and again checking the baby, the mother is able to get dressed and go home.
Almost without exception, patients experience far less discomfort from the test than they had expected. Any discomfort felt is usually similar to that felt for a simple blood test. It is recommended that for the rest of the day things are taken quietly, but after this to return to a usual routine.
Could There Be Any Problems Afterwards?
It is very rare to have any problems at all after the test. Occasionally there may be some mild discomfort due to a little bruising under the skin. If there is any loss of blood or water from the vagina or any pain after the amniocentesis you should consult your obstetrician.
What Tests Are Performed On The Fluid Taken?
An estimate of the amount of a particular protein (alpha feto-protein). There are usually large amounts of this protein present in the fluid if a baby has a defect called Spina Bifida.
The second test is carried out on the cells which are present in the fluid. The chromosomes of these can be examined and in this way it is possible to detect whether the baby has Downs Syndrome or any other less common chromosome problems. The sex of the baby is also determined.
