Amniocentesis Procedure and Related Tests

The Amniocentesis procedure involves a needle being inserted through the abdominal wall into the amniotic sac in the uterus. A sample of the amniotic fluid can then be withdrawn and examined in the laboratory. Before the needle is inserted, an ultrasound scan is done to show the position of the placenta and the baby so that neither is damaged by the needle.


The procedure is used in the first half of pregnancy, usually at about fourteen to sixteen weeks to detect downs syndrome and spina bifida babies.

Down’s Syndrome is diagnosed by taking some of the baby’s skin cells, contained in the amniotic fluid, and growing them on a suitable culture medium. The body cells of Down’s Syndrome babies have a different chromosome count from normal individuals and this can be amniocentesisdemonstrated in the nuclei of the growing cells. It takes between two and four weeks to complete the test. As the culture does not always take the first time round, the test might have to be repeated.

The risk of having a Down’s Syndrome baby is about one in a thousand in women up to the age of thirty-five. For women over this age, and for women who have a family history of the condition or who have previously had one Down’s Syndrome child, the risk is greater. By the time a woman is forty the risk of having a Down’s Syndrome child is one in eighty.

For this reason, it is now recommended that all women over the age of thirty-five should have an amniocentesis to exclude Down’s Syndrome. Although the amniocentesis procedure itself carries a slight risk of causing a miscarriage, the advantage of knowing that the baby is not going have Down’s Syndrome outweighs this risk in most women with an increased chance of having one.

Spina Bifida

This condition can be detected by an ultrasound examination alone but, in most cases, it will be necessary to confirm the diagnosis by examining the amniotic fluid for alpha fetoprotein. The test can be carried out quickly, unlike the test for Down’s Syndrome, and the result is available within a few days.

Alpha feto-protein can also be detected in the blood and it is hoped that by doing this blood test on all pregnant women those with a possible spina bifida fetus will be picked up. An amniocentesis to detect spina binda is only done at present where the blood test is positive, there is a family history or where the woman has already given birth to a spina bihda baby. Before having an amniocentesis to detect mongolism or spina bifida, it is important that the woman concerned decides that if the test is positive, she will be prepared to have a termination.


Amniocentesis is used in late pregnancy principally to detect bile pigment or lecithin and sphingomyelin in the fluid. Bilirubin is the bile pigment present in the fluid when the fetus has become severely jaundiced as a result of rhesus incompatibility. When the level is high steps must be taken to lower the level or deliver the baby.

The blood vessels in the placenta join together and emerge from the centre of its fetal surface as two arteries carrying blood from the fetus to the placenta and one vein carrying blood the other way. These three blood vessels are surrounded by a special jelly-like substance and enclosed by an outer membrane to form the umbilical cord, which stretches out from the placenta to enter the fetus at the site of its future navel.

There, the blood vessels from the placenta ioin up with those of the fetus. The feto-placental circulation is thus completed. The fetal heart can now pump blood through the umbilical arteries to the placenta and receive blood back from it in the umbilical vein. At the find of pregnancy the umbilical cord is about 50 cm (20 in) long and is the vital link between the baby and its placenta.

Astronauts walking in space refer to the pipe which connects them to the space craft as their umbilical cord. The comparison is very apt, since, in both cases, the individual is dependent upon the oxygen brought to it through the connecting link.

Lecithin and Sphingomyelin

These substances are present in the baby’s lungs and enable the lungs to open up after delivery when the baby takes its first breath. A lack of lecithin and sphingomyelin leads to great difficulty in breathing and the baby sometimes dies of the condition known as the respiratory distress syndrome or hyaline membrane disease.

Premature babies may lack the necessary lecithin and sphingomyelin to enable their lungs to function properly. For this reason, when a woman threatens to go into labor prematurely, or when it is thought advisable to deliver the baby early, a sample of amniotic fluid will often be taken and examined for these substances. If they are not present in adequate amounts, steps will be taken to increase the levels by medical means, or to delay delivery.

In addition to these tests, the following are also usually carried out:


The foetoscope is a large needle that acts as a telescope and allows the operator to examine the baby inside the amniotic sac. The procedure is in the early stages of development at present, but holds great promise for the future. In addition to enabling abnormalities such as hare lip and cleft palate to be diagnosed, it may also be possible to obtain a sample of the fetal blood and test it for congenital blood diseases.


This is a new antenatal test to see if the baby is getting adequate oxygen from the placenta. Using a special ultrasound detector, placed on the mother’s abdomen over the region of the fetal back, the fetal heart sound can be picked up and recorded on a moving paper strip.

A normal fetal heart rate and rhythm indicates that all is well with the baby. Sometimes another recording device is placed on the mother’s abdomen at the same time and this records when the uterus contracts.

The effect of the contractions on the fetal heart rate and rhythm can then be seen on the graph paper. Although not generally used as yet, this sort of test is likely to become more common in the future especially as ultrasound machines become more sophisticated. Thus, it is already possible to use ultrasound to observe and measure the breathing movements which every fetus makes inside the womb during pregnancy. Although in the unborn baby breathing does not take place in the same way as it does after birth, the breathing movements on the fetal chest wall are thought to be an important aspect of normal fetal activity. An absence or lessening of these movements may indicate that the fetus is suffering from a lack of oxygen.