Letters

Increased parity and risk of trisomy 21

BMJ 1997; 314 doi: https://doi.org/10.1136/bmj.314.7096.1760a (Published 14 June 1997) Cite this as: BMJ 1997;314:1760

Study measured prevalence of Down's syndrome at birth, not incidence

  1. P O D Pharoah, Heada
  1. a Department of Public Health, University of Liverpool, Whelan Building Quadrangle, Liverpool L69 3GB
  2. b Department of Neonatology, Shaare Zedek Medical Centre, PO Box 3235, Jerusalem, Israel 91031
  3. c Health Division, Israel Central Bureau of Statistics, Jerusalem

    Editor—Michael S Schimmel and colleagues state that the incidence of Down's syndrome is 1.46 per 1000 live births when they mean the prevalence at birth.1 The accompanying commentary claims that the authors used prevalence as a proxy for incidence on the grounds that there were no terminations because the community eschewed prenatal diagnostic services. The authors do not make this claim, but even if they did it is not valid. Assuming that there were no terminations for Down's syndrome, it is still valid only to determine the prevalence at birth because there is no means of knowing how many conceptuses with Down's syndrome were spontaneously aborted. A disproportionate number of spontaneous abortions are chromosomally abnormal and a significant proportion of conceptuses with Down's syndrome are spontaneously aborted. Hypothetically, as a woman's parity and age increase she could become less efficient at “recognising” and spontaneously aborting a conceptus that has Down's syndrome. If this were so the prevalence at birth cannot be a proxy for incidence.

    The authors may have shown an increase in the birth prevalence of Down's syndrome with increasing parity which is independent of age, but they have not provided any evidence for a parity effect on incidence. The distinction between incidence and prevalence is paramount when considering the aetiology of any condition.

    References

    1. 1.

    Authors' reply

    1. Arthur I Eidelman, Professor of paediatricsb,
    2. Pnina Zadka, Senior epidemiologistc
    1. a Department of Public Health, University of Liverpool, Whelan Building Quadrangle, Liverpool L69 3GB
    2. b Department of Neonatology, Shaare Zedek Medical Centre, PO Box 3235, Jerusalem, Israel 91031
    3. c Health Division, Israel Central Bureau of Statistics, Jerusalem

      Editor—We thank P O D Pharoah for the opportunity to clarify our presentation on the incidence of Down's syndrome. As we noted in the article, there was an increased incidence in the birth of infants with Down's syndrome in mothers with high parity in a population in which termination of pregnancy was essentially zero. In addition, to our knowledge there are no data to suggest that increasing parity affects the rate of spontaneous abortions. Thus, we think that our data truly reflect the incidence at birth, though not necessarily the incidence at conception.

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