Authors did not define criterion for case selection

EDITOR—Hawthorne et al claim to show that women with diabetes have a much more unfavourable outcome of pregnancy in England than Norway.1 But what does their study really show?

Their criterion for case selection (“diabetes”) was not defined. But the prevalence differs hugely between the countries: 1 in 335 pregnant women in northeast England and 1 in 90 in Norway were reported to have diabetes. This suggests that selection in the two countries was based on different clinical criteria. In northeast England most women included in the northern diabetic pregnancy survey were taking insulin before pregnancy, and all cases were confirmed by the clinicians and from the patient record.2 The data from Norway are from the centralised medical birth registry, and the possible pitfalls of this are illustrated by experience in Scotland.

I checked national registry listings of diabetes as part of the protocol for the SIGN (Scottish Intercollegiate Guidelines Network) guideline on management of pregnancy.3 Some women recorded as having diabetes did indeed have this, but some had only impaired glucose tolerance; some had had a glucose tolerance test but the result was normal; some …