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

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 …