Diabetes and Congenital Abnormalities

Because there are so many things to know, it is hardly surprising that all of us don't know much. Bandolier had no inkling, for instance, that mothers with diabetes had a higher risk of having children with congenital abnormalities. A new study [1] estimates the frequencies, and tells us some of the problems.

Since 1982 in Hungary it has been compulsory to register all congenital abnormalities. It is a thorough and comprehensive system. Cases were children identified with serious congenital abnormality from 1980 to 1996, with, for each, two newborn children without congenital abnormality as controls. In each case questionnaires were sent to mothers about maternal health, drugs, and pregnancy complications, and district nurses questioned all non-responding case families and control families. Diabetes was defined as use of insulin during or before the first trimester.

There were 22,843 cases, of whom 63 (0.3%) were born to mothers with pre-gestational insulin-treated diabetes. Among 38,151 controls, there were 50 diabetic mothers (0.1%). Before and after adjustment for possible confounding factors, the odds ratio was 2.1 (95% CI 1.5 to 3.1). No mothers had taken antiepileptic drugs.

The 63 cases comprised a range of different serious congenital abnormalities. The largest group was cardiovascular, followed by multiple congenital abnormalities, and hypospadias; together these comprised 36 of the 63 cases, and for most specific conditions there were four or fewer cases.

The organisation of a countrywide register like this is an enormous undertaking, as is the organisation of surveillance and analysis. Yet even such an undertaking identified only 63 congenital abnormalities in children of diabetic mothers. Our best guess is that the rate is doubled in diabetes across a basket of different serious congenital abnormalities.

The paper notes that the four largest studies published previously comprised 25, 28, 59, and 75 congenital abnormalities in children of women with pre-gestational insulin-dependent diabetes. What we cannot say, because of the nature of the case-control design, is what is the absolute risk of a serious congenital abnormality for women with pre-gestational diabetes. All we can say is that it is twice as high as for women without diabetes.

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