Along with the regular pregnancy testing you have to endure, there is another more invasive set to collect. Dr Lee guides you through the maze

In all pregnancies there is a basic set of 'invasive' investigations that usually occur at set stages of gestation and are designed to ensure the continuing good health of the baby and mother.

A major fear for any parent-to-be is the possibility of congenital abnormality in their forthcoming baby. Following the initial 'booking in' appointment or from a prior knowledge of the medical or obstetric history, certain situations may be found that need further investigation and observation.

Antenatal services aim to diagnose congenital abnormalities as early as possible. If an abnormality is detected early enough, the couple may be offered the choice of a termination.

At the moment, screening for congenital abnormalities is mostly concerned with the detection of malformations of the central nervous system (like Spina Bifida) and abnormalities of chromosomal origin (like Down's Syndrome).

The special tests

Amniocentesis
This test is usually carried out at 16-18 weeks to detect abnormalities in the foetus. However, it is now possible to get a good sample of amniotic fluid safely at 12-13 weeks and, in some specialist centres it may be performed as early as 10 weeks.

This test is usually offered to older women (usually in their late 30s) as this age group have a higher risk of having a Down's Syndrome baby.

Amniocentesis is also offered if there is a family history of genetic abnormalities, or a positive result from certain of the screening tests done in early pregnancy.

In this test a needle is inserted through the wall of the abdomen and womb, under ultrasound guidance, to obtain a sample of the amniotic fluid (in which the baby is floating). Cells from the foetus are floating in the amniotic fluid and these are needed for testing.

The results are usually available after a few weeks. There is a small chance of miscarriage (less than one per cent) following this test.

Chorionic Villus Sampling (CVS)
This test also checks for genetic abnormalities (but not Spina Bifida). CVS is usually performed at 10-12 weeks. Once again, foetal cells are required, but they are obtained from the developing placenta.

A minute sample of tissue is taken with a needle, using ultrasound to control the route. A preliminary result is usually available after a few days. This test also carries a risk of miscarriage (0.5 per cent - 2 per cent).

The Triple Test
Down's Syndrome is more common in women over 35. However, half of the babies with this condition are born to women under that age.

Although the risks of having a Down's Syndrome baby under 35 are less, the number of babies born to mothers in this age group is very much greater. The amniocentesis and CVS tests are unsuitable to be used as screening tests for all pregnant women as they are invasive procedures.

The Triple Test is a simple screening test carried out in early pregnancy which assesses the concentrations of three chemicals (alpha-fetoprotein, hCG and unconjugated estriol) in the maternal blood.

Such screening can, therefore, be performed in all women irrespective of age, but is usually offered to those thought to be at higher risk.

A Quadruple test, which adds in a check for levels of the hormone inhibin A, and even a Penta Screen, which also measures Invasive Trophoblast Antigen have also been developed but are not yet available in all areas.

Results from all these tests are used together with maternal age, gestational age of the fetus, whether or not there are twins or more, to determine overall risk.

Nuchal Thickness Scanning
A screening test performed at about ten weeks that looks specifically at the thickness of the skin at the back of the neck of the foetus. This is done by taking measurements through ultrasound imaging.

This can act as an aid to the prenatal diagnosis of chromosomal defects, particularly Down's Syndrome; the thicker the fold, the higher the risk of the baby having chromosome defects.

Sensitivity varies widely according to the skill of the ultrasound operators and the equipment used. Those experienced in using the test claim that sensitivity will improve and other ultrasound indicators of Down's Syndrome are being added.

These include the shape of the head and the length of the bones, for example.

Alpha-fetoprotein AFP
In mid-pregnancy (14?16 weeks) the concentration of a foetal protein (Alpha-fetoprotein, AFP), that has been taken into the mother's blood from the amniotic fluid can be measured from a blood sample.

In defects of the baby's spine and head development (open Spina Bifida and Anencephaly) there has been a considerable escape from the defect and therefore there is a much higher than expected level of the AFP in the mother's blood.

The test has a high predictive value. If this is found, a special ultrasound scan may be performed to examine the spine and head carefully at 16-17 weeks of gestation.

There are various conditions that can produce false positives and these conditions have to be considered.

As these early, antenatal detection screening tests develop and become more efficient, there will be a similar increase of awareness and preparation possible that will allow parents and carers to make vital and important decisions about the baby.