Letters

Factors influencing relative weights of placenta and newborn infant

BMJ 1997; 315 doi: https://doi.org/10.1136/bmj.315.7121.1542 (Published 06 December 1997) Cite this as: BMJ 1997;315:1542

Maternal haemoglobin and blood pressure should have been regarded as continuous variables

  1. Philip Steer, Professora
  1. a Academic Department of Obstetrics and Gynaecology, Charing Cross and Westminster Medical School, London SW10 9NH
  2. b Women and Infants Research Foundation, Department of Obstetrics and Gynaecology, University of Western Australia, King Edward Memorial Hospital, Perth, Australia

    Editor—Williams et al suggest that maternal anaemia (which they define as a haemoglobin concentration of <110 g/l at any time in pregnancy) is related to birth weight, placental weight, and the placental weight to birthweight ratio.1 They used multiple regression and logistic analysis to examine the relation between the variables.

    Maternal haemoglobin concentration should not be used as a dichotomised variable in this type of analysis, because its relation with birth weight is U shaped.2 Thus any single cut off will misrepresent the true correlation. Maximum birth weight is associated with a maternal haemoglobin concentration of 95–105 g/l, which is why anaemia in their analysis (including as it does the physiological optimum for birth weight) is associated with a higher birth weight than lack of anaemia (high haemoglobin concentrations do not indicate only absence of anaemia but can be due …

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