Letters

Prenatal screening for trisomy 18: Should not be contemplated

BMJ 1994; 308 doi: https://doi.org/10.1136/bmj.308.6926.471 (Published 12 February 1994) Cite this as: BMJ 1994;308:471
  1. T Davies

    EDITOR, - K Spencer and colleagues' claims for prenatal detection of trisomy 18 by measurement of maternal serum (alpha) fetoprotein and free β human chorionic gonadotrophin concentrations are impressive.1 Detection of 50% of cases for a false positive rate of only 1% seems to compare favourably with the detection rate for Down's syndrome when similar techniques are used, which is 70% for a false positive rate of 5%. Unfortunately, the authors fail to emphasise the importance of the relative incidence of the two conditions at birth before concluding that screening for trisomy 18 should be introduced.

    The natural incidence of Down's syndrome at birth is approximately 12.6/10 000 births.2 Among 10 000 pregnant women a 70% sensitivity would result in 8.8 cases being detected at the …

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