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First trimester screening accuracy

A First trimester screening has moved forward hugely in recent years. It is now usual to assess the risks of conditions such as spina bifida or Down's syndrome.

The overall calculation of risk may involve more than one screening method such as serum, urine and ultrasonography in addition to the woman's age.

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In the first trimester a number of serum markers are used in screening for Down's syndrome. Maternal age remains an indicator. A woman of 25 has a 1:45 chance of being screen positive whereas a woman of 44 has a 1:2 risk.

The double, triple and quadruple tests can all be done between 15-16 weeks with a detection rate of 57, 69 and 76 per cent respectively. Each has a false positive rate of 5 per cent.

The nuchal transluency (NT) test and combined NT and biochemical markers test can be done between 10-13 weeks with a detection rate of 78 per cent and false positive rate of 8.3 per cent for the former and 85 per cent detection rate and 5 per cent false positive rate for the latter.

The integrated test – involving NT and biochemical markers plus alphafeto protein, uE3 and inhibin A – is a two-stage process.

Stage one should ideally take place at 12 weeks but can be done between 10-13 weeks.

The second test should be done at 15-16 weeks but can be done up to 22 weeks. It has has a detection rate of 95 per cent and false positive rate of 5 per cent.

The availability of tests depends on the area in where a woman lives.

Michelle Lyne, is lecturer in midwifery, City University,

St Bartholomew School of

Nursing and Midwifery

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