30-WEEK VISIT: OESTRIOL
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Oestriol is a substance in women's urine which rises in volume as the pregnancy goes on after twenty-eight weeks. Its measurement is used as an alternative or complement to HPL. It is said to be another means of measuring whether or not the placenta is doing its job of nourishing and protecting the foetus. It too has a low predictive value though there are those who argue that oestriol levels in blood can give more accurate results.
There was a time when all urine tests were based on urine samples collected over a complete 24-hour period. Today, a urine test called an oestriol/creatinine ratio is said to be accurate on the basis of one sample only. It is claimed by its supporters to be the most accurate biochemical measure available of foeto/placental function. But one study reported that while it only missed about ten per cent of women 88 who later proved to have problems, it also picked up over fifty per cent who later proved to have no problems at all. As with some of the screening tests of early pregnancy, tests like this cause considerable anxiety. Used selectively among women considered to be 'at risk' the benefits may outweigh the disadvantages, but their predictive value is too low for them to be beneficial to the majority of pregnant women.
Biochemical tests may be most effective when taken together. When oestriol and HPL are both low there is a two-thirds probability that something is going wrong. But both tests are only valuable if a number of measures are taken over a period of weeks to plot a rate of change in the HPL and oestriol levels. They also require a fairly accurate knowledge of dates because the level of these substances varies with the stage of pregnancy. A recent survey of biochemical tests concluded:
In general, it may be said that in practice [biochemical] tests of placental function have not fulfilled the expectations of their originators partly due to the variety of [complications] found in pregnancy and partly due to the overlap of values found in normal and abnormal pregnancies.
This is important to remember if attempts are made to persuade you to have an induced delivery on the basis of results from these tests.
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Women's Health
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