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Pre-natal testsPre-natal Tests

Every mother-to-be wants a healthy, normal baby and doctors now have a battery of sophisticated tests which they can use during the pregnancy to keep a check on the health of the developing foetus.

When a baby is born, the mother's first question is usually 'Is my baby all right ?' Pre-natal tests which screen for specific types of abnormalities can help dispel some of the parents' worries before the birth.

What they can't do, though, is guarantee that a baby will be born normal and healthy. However, forewarned means forearmed, so if a foetus is identified as having an abnormality, any special care ortreatment it may need -- such as an incubator -- can be ready and waiting when it is born. In some cases, it may even be possible for doctors to treat the foetus in the womb, for instance by giving vital blood transfusions.

There are also some cases where the early diagnosis of serious foetal abnormalities can give an expectant mother the option of terminating the pregnancy.

ROUTINE TESTS

One of the first of the routine tests in pregnancy is the blood test. It is used to identify the mother's blood group and to see if she is anaemic, which means she has a lack of haemoglobin in her blood cells. This is a common problem which, left untreated, can adversely affect her baby's development. However, it's easily treated and this is one of the reasons why the blood test is among the first to be made.

A blood sample can also reveal if she is likely to be immune to rubella, or German measles, a relatively harmless disease except in pregnancy, when it can severely damage the foetus. Rarely, she may be tested for hepatitis or the HIV virus, or for any other sexually transmitted diseases.

An expectant mother will also be asked to provide regular urine samples throughout her pregnancy. The samples are tested for sugar, a sign of diabetes, or protein, which is associated with pre-eclampsia -- a life-threatening condition which can develop in later pregnancy.

Routine ultrasound scanning is very safe and allows medical staff to 'look' inside the womb to check on the development of a foetus. It involves the sending of very high frequency sound waves into the womb. A picture of the inside of the uterus can then be built up by analysing the echoes that are reflected back. A routine ultrasound is normally carried out between 16 and 22 weeks of pregnancy.

EARLY INFORMATION

The ultrasound test can help confirm the gestational age of a foetus, which is known to be estimated wrongly by a quarter of all expectant mothers. It should reveal whether a woman is expecting more than one baby, check whether the baby seems to be growing properly and that the placenta is in the right position, and screen for obvious abnormalities in the nervous system, kidneys, heart, intestines or skeleton.

 

ADDITIONAL TESTS

Pre-natal tests for other foetal abnormalities can be divided into two categories: screening tests that estimate the chances of a foetus having a specific abnormality, and diagnostic tests which will establish whether in fact it does.

The screening tests can't give definite answers, only estimates of risk. They are based either on analysing the mother's blood for certain biological 'markers', or on specialized ultrasound scans.

Pre-natal biological markers are hormones or other substances in a pregnant woman's blood. At certain levels, these can indicate that she is at increased risk of carrying a baby with, for example, neural tube defects -- a developmental failure affecting the baby's spinal cord or brain -- or the chromosomal disorder, Down's syndrome, which results in mental handicap.

There are several biological markers which can be used to screen for Down's syndrome. Individually, they are not as reliable as when taken together and the results used in conjunction with the mother's age. These combined tests are usually performed at around 13 to 18 weeks of pregnancy and have an 80 per cent success rate of detecting a Down's pregnancy. They will also reveal the risks of a foetus having neural tube defects as well as many other less common abnormalities.

ARE SCANS EFFECTIVE?
In skilled hands, ultrasound scans carried out as early as 11 weeks into pregnancy can detect 85 per cent of babies with chromosomal abnormalities like Down's, The nuchal transparency scan, as it's called, involves measuring the size of a membrane behind the neck, which shows up as a black space. The result of the scan is then computed with the woman's age to assess the risk of her baby having a chromosomal disorder.

Those at greatest risk of carrying a foetus with any of these types of abnormality are then normally given the chance to have a diagnostic test. This is a much more definitive test which will establish whether the foetus really does have the abnormality in question. The reason why all women are not simply offered the clear-cut diagnostic tests straight away is that they are more invasive and there is sometimes a small chance that they could provoke a miscarriage.

DIAGNOSTIC TESTS
The only 'way to diagnose for certain whether a foetus has a particular abnormality is to obtain some of its cells for laboratory analysis. There are different ways of doing this but the most common are amniocentesis and chorionic villus sampling (CVS).

These tests are normally offered to women in their late thirties to screen for Down's syndrome because they stand an increased chance of conceiving a Down's baby. It is also offered to those found to be at high risk of having a Down's baby after preliminary screens, or to those with a family history of other chromosomal or genetic abnormalities.

AMNIOCENTESIS
Amniocentesis is normally carried out at 15 to 16 weeks of pregnancy. Guided by ultrasound, a fine, hollow needle is inserted through the mother's abdomen into the womb and into the sac of amniotic fluid which surrounds and cushions the growing foetus. A local anaesthetic may be used, though it's not usually needed as the process is not particularly painful.

A small sample of the fluid, which contains cells shed from the developing foetus, is then drawn off. The foetal cells floating in it are isolated and analysed.

If the test has been carried out to detect neural tube defects, it takes about a week before the results are obtained. But, if the test is used to screen for Down's, results take three to four weeks to come through, because the cells must first be cultured before they can be analysed. There is a slightly increased risk of miscarriage as a result of the amniocentesis test.

THE CVS TEST

CVS ( chorionic villus sampling ) is more risky than amniocentesis, possibly provoking miscarriage in one in 100 pregnancies. It can be carried out much earlier in the pregnancy though -- ideally at 10 to 12 weeks into pregnancy.

CVS can detect chromosomal abnormalities but not all types of neural tube defects. The test involves removing cells from the chorionic villi -- finger-like tissues forming part of the placenta.

A fine, flexible tube is inserted into the womb through the cervix under ultrasound guidance and a piece of the tissue sucked out. In later pregnancy, it's considered too risky to carry out the test through the cervix, so it's done through a small puncture in the mother's abdomen. Results can be obtained faster than with amniocentesis, too -- usually within a few days. But CVS is more painful and requires more skill on the part of the doctor than the amniocentesis test.

Cordocentesis is a special test used to obtain a blood sample from the foetus. Guided by ultrasound, the doctor inserts a needle through the mother's abdomen and into the umbilical cord, from which a sample of blood is drawn off.

The blood sample is used to rule out the blood disorder rhesus disease; to assess how well the baby's kidneys are working; to check for certain rare inherited abnormalities and to check if it's getting enough oxygen in its blood supply if it is undersized for its age.

INSIDE VIEW

Foetoscopy, where the foetus is directly viewed or a sample of its blood or tissue is taken for analysis, has now largely been replaced by ultrasound. it involves inserting a very slim telescope-like instrument into the womb to look at the foetus and nowadays it is mostly reserved for identifying certain defects which can characterize particular life-threatening developmental disorders.

     
     

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