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Miriam's casebook - Testing for abnormalities

At 26, Daniella was particularly keen to have her babies while she was young and fit. After Daniella had missed two periods and a home pregnancy test had proved positive, she and her partner Will visited their doctor, who referred them to the antenatal clinic at the local hospital. They both found this visit off-putting, particularly when they realized how many screening and diagnostic tests they would be offered.

Why screening is offered

Abnormalities?
© DK

Today's technology allows doctors to investigate the health of the fetus before birth at earlier and earlier stages. Like Daniella, most pregnant women are now offered routine screening tests. These sometimes lead to much more invasive diagnostic tests for confirmation with little discussion of the implications of the tests and what they might discover about the health of a baby in the womb.

At the clinic Daniella and Will were given a list of routine screening tests that would be carried out during Daniella's pregnancy. As a young, healthy first-time mother, Daniella hadn't expected to be screened for fetal abnormalities. They both found the idea frightening and it started them thinking about the implications of screening tests. They were both keen to have a natural birth and were resistant to the technological approach they met at the hospital.

Routine screening tests

Daniella and Will were told that she would start having tests to screen for abnormalities in her baby at 11-12 weeks with the first ultrasound scan. Then at 14-plus weeks, a blood test (known as the serum screening test,) would be taken to look for Down's syndrome, spina bifida, and hydrocephalus. At 18-22 weeks, she'd have her second ultrasound scan, possibly after an amniocentesis (a diagnostic test, usually carried out at 14-18 weeks) if either screening test suggested a risk of abnormality in her baby.

I explained to Daniella that a screening test is by definition a blunt instrument. It doesn't give any precise information. It can do no more than pick up a tendency for something to happen. The way doctors express this tendency is in terms of probability.

So if a blood test gave a one in 500 chance of the baby having Down's syndrome, it would mean that if Daniella had 500 babies, one of them would have Down's. By any criteria this is a very small risk, but as Daniella said, any risk of Down's syndrome seemed a big risk to her. Like all mothers, she wanted the test to come back entirely negative. As I pointed out, on the scale of probability, one in 500 is exceedingly low. There'd be little cause for concern until the risk rose, to say, one in 250, the probability threshold at which amniocentesis would be offered to every pregnant woman. The next step would be a precise diagnostic test to detect a specific abnormality. This is normally only done if a screening test is positive.

How diagnostic tests work

Such tests are precise enough to give the answer “yes” or “no”; “present” or “absent”; “normal” or “abnormal”. Unfortunately, they're quite invasive, and the most widely used, amniocentesis, involves a specimen of amniotic fluid containing cells from the baby being drawn out of the uterus - a delicate operation requiring skilled guidance with ultrasound. The cells are then examined for chromosomal or genetic damage in a specialist laboratory. I told Daniella that amniocentesis itself carries a risk of miscarriage of about two in 100 (two per cent).

I explained that in most centres, the definitive test takes three weeks, as it takes that long for enough of the baby's shed cells to multiply sufficiently to be safely analyzed. The newer polymerase chain reaction (PCR) results, however, are available much sooner and are regarded as being highly reliable. Daniella then asked what would happen if amniocentesis confirmed Down's. I explained that she and Will would then be counselled about whether or not to terminate the pregnancy. Of course hearing this upset Daniella as she had never even thought about the possibility of terminating the pregnancy.

Other available tests

I went on to explain that a nuchal scan, which can be done as early as 11 weeks, would alert Daniella and Will to the possibility of a chromosome defect in their baby. This would be followed immediately with a diagnostic test, such as chorionic villus sampling (CVS), which can be done much earlier in pregnancy than amniocentesis and results can be obtained in as little as 24-48 hours. Like amniocentesis, CVS carries a small risk of miscarriage.

Screening tests aren't compulsory, they're optional. I suggested to Will and Daniella that they asked to see the obstetrician in charge of the clinic to talk further, before deciding their approach to tests. In the event, they had a healthy baby girl, born at their local hospital under the care of a team of local midwives, and the family is thriving.

Checklist of specialist tests

Screening tests

  • Nuchal scan: An ultrasound scan at 11-14 weeks that screens for high risks of chromosomal defects (see Nuchal scan).
  • Serum screening test: A sample of the mother's blood at 14-20 weeks screens for hormone levels that indicate a higher risk of Down's syndrome (see Serum screening).

Diagnostic tests

  • Chorionic villus sampling (CVS): Cells from the developing placenta are examined at ten to 12 weeks to check the baby for chromosomal abnormalities.
  • Amniocentesis: Fetal cells from the amniotic fluid are removed at 14-18 weeks and checked for chromosomal abnormalities such as Down's syndrome.
  • Cordocentesis: Fetal blood from the umbilical cord is tested for abnormal chromosomes or infection.
  • Doppler scan: A special scan that looks at the blood flow between the placenta and your baby through the umbilical cord (see Doppler scan).

Miriam's top tips

  • Find out as much as you can about the tests you are likely to be offered during your pregnancy. Discuss their implications with your partner and your healthcare professional.
  • Ask your partner to accompany you to scans and routine tests. It's comforting and reassuring to have another person to support you at these times.
  • Take advice at every stage and seek a second opinion if you feel you need further clarification.

Posted 16.11.2010

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