Consultant Obstetrician & Subspecialist in Maternal Fetal Medicine

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Mothers Experiences

Prenatal Diagnosis

Tests to Screen the Baby

Most babies develop normally, but unfortunately about 2% have a significant abnormality. Screening tests can show if there is an increased chance that your baby has a problem, such as a neural tube defect, Down's syndrome or other, rarer abnormalities. However, none of the tests available today are completely accurate. Agreeing to a screening test does not mean you have decided what to do if a problem is found. Some couples simply prefer to know in order to be prepared, whilst others may choose to terminate their pregnancy.

18-22 week scan This is a general scan of the fetal anatomy. This may show an isolated anatomical defect or a combination of anatomical abnormalities. Occasionally both single and multiple defects will be due to either chromosomal eg Down’s syndroem or genetic causes eg cystic fibrosis.

Cardiac Scan: Abnormal hearts may be isolated findings in the fetus and simpy due to an anatomical abnormality. However, they too may be due to a chromosomal or genetic problem.

If abnormalities are seen at either the 18-22 week or cardiac scan we will discuss with you if there may be a chance of a chromosomal or genetic cause. If so we may then offer you further diagnostic investigations such as amniocentesis. Not all genetic syndromes may be diagnosed by amniocentesis.

Neural Tube Defects eg Spina Bifida. An opening anywhere along the baby's spine can result in paralysis of the lower limbs or a collection of water on the brain (hydrocephalus). An important warning sign is raised AFP (alphafetoprotein), measured in your blood which may be taken for the Down's syndrome test (see below). The diagnostic test is a detailed ultrasound scan.

Down's Syndrome. Babies with this condition have an extra chromosome 21 , and are at increased risk of heart and bowel problems and learning disabilities. The overall risk increases for older mothers. The risk is best assessed by one or a combination of screening tests.

1. Blood tests - measure 'serum markers' and use other factors (your age, weight and scan dates) to work out your chance of having a baby with a problem. This includes the Triple test and is taken at around 16 weeks.

2.Nuchal Translucency Scans - estimate the fluid at the back of the baby's neck at around 12 weeks. The presence of extra fluid increases the chance that there is a chromosomal or heart problem.

If the risk calculated from the results of the Down's tests is less than the recommended cut-off limit (I :250, or 0.4%), it is designated 'low risk'. But it is important to be aware that none of the screening tests are failsafe; they detect only about 60 - 80% of all cases. If the risk is greater than the I :250 cut-off limit, the result is considered 'high risk' - but it is still most likely that your baby does not have Down's Syndrome or any other chromosomal abnormality. However, you will be offered an additional test to make an accurate diagnosis, such as Amniocentesis or Chorionic Villus Sampling (CVS).

Diagnostic Tests for Chromosomal Abnormalities

These are done using ultrasound guidance to reduce the risk of injury to the baby. They are offered when the result of the screening test is reported as 'high risk' to anyone with a family history of an inherited problem, or as a result of scan findings.

Amniocentesis: a fine needle is passed through the wall of the abdomen and womb to remove a small amount of the fluid that surrounds the baby. The sample is tested in the laboratory to check the baby's chromosomes (genetic makeup). It is normally performed at around 16-18 weeks. Amniocentesis has a miscarriage risk of approximately I in 100.

CVS (Chorionic Villus Sampling): A needle is used to remove a small sample of tissue from the placenta (afterbirth). This can be done earlier than amniocentesis. The chance of miscarriage is higher than with amniocentesis.

The laboratory will use either full karyotype or rapid testing methods (PCR/FISH). Full karyotype takes 2 to 3 weeks and looks at all chromosomes. PCR/FISH only looks for specific chromosomes but takes I to 2 days.

Please note: Results may show chromosomal problems other than the one being tested for, e.g. Edward's or Turner's syndrome. Very occasionally, a result cannot be obtained from the sample taken and the test has to be repeated.

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