Amniocentesis is an antenatal test performed by drawing off some of the amniotic fluid surrounding the foetus, for analysis.
In the first half of thethis test can be performed to check whether certain congenital disorders – such as Down’s syndrome – are present. There is reason to perform amniocentesis when relatives or a previous child of the parents are afflicted with some type of congenital disorder. Sometimes the age of the woman can be a reason, because the risk of bearing a child with Down’s syndrome increases with the increasing age of the mother.
If a woman thinks the possibility of a genetically determined problem may arise during, she may go to a genetic counsellor. It is certainly not unwise to get his advice even before she tries to get pregnant. Such a doctor can take a medical history of the woman and her partner, detect any possible genetic incompatability and try to estimate the risks, and can provide information about the possibilities of screening during pregnancy. She then can decide whether she wants to give up the idea of becoming pregnant, take the risk of having a handicapped child or whether she wants an amniocentesis during pregnancy to be sure the child is not affected. In general this means that on a negative outcome of the screening (the baby is affected), an abortion can be considered.
When an amniocentesis is done, first an ultrasound scan is necessary to check for the position of the placenta and foetus so that they can be avoided. Then a hollow needle is inserted through the abdominal and uterine wall into the amniotic sac and some of the fluid is drawn off.
The fluid can then be examined. Sometimes it is necessary only to search for certain chemical substances. An elevated amount of AFP (alphafoetoprotein) for example can make it likely that the baby has spina bifida.
In addition, it is possible to examine foetal cells also present in the amniotic fluid for chromosomal aberrations. For certain genetic abnormalities such as haemophilia* it is only necessary to determine the sex of the baby, as usually only male offspring can get this disorder from their parents. An amniocentesis can be performed from the sixteenth week of pregnancy onwards, and some tests last up to several weeks. A negative outcome (the baby appears to have the abnormality tested for) means that an abortion sometimes has to be performed at the twentieth week of pregnancy.
At this time the mother has usually felt her child moving already. This may pose a real psychological burden to her.
These problems can sometimes be prevented when a new method of antenatal screening is used, called chorionic villus sampling, in which small samples of the developing placenta are examined. The villi can be obtained easily by inserting a catheter through the cervix of the uterus. Because the placenta contains the same genetic material as the baby, it is also possible to check for certain genetic abnormalities of the child. The advantage of this method is that it can be performed as early as the eighth week of pregnancy.
In the second half of pregnancy, however, amniocentesis still has its value. Amniotic fluid can for example be examined to determine whetherincompatability* has arisen. Another reason for amniocentesis arises when the doctor wants to determine whether the lungs of the child are mature enough for him or her to survive in the outside world. This maturity is measured from the amount of certain enzymes in the amniotic fluid.
Such information can for example be necessary when the child has stopped growing and the gynaecologist wants to estimate the risk of premature delivery (by Caesarean section) as opposed to letting the pregnancy carry on to full term.