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Editor's note References 22–30 of the paper by Hannaford et al were incorrect, and each correlate with the number below in the text. Thus reference 22 in the reference list corresponds with reference 21 in the text, and so on. This correction appears online [adc.bmjjournals. com/cgi/content/full/fetalneonatal;81/3/F162/DC1] The journal would like to apologise for this error.