The risk of birth defects increases four-fold if the pregnant mother has diabetes, researchers say.
The Newcastle University study, published in the journal Diabetologia, analysed data from more than 400,000 pregnancies in North East England.
The risk of defects such as congenital heart disease and spina bifida were increased.
National guidelines already recommend having good control over blood sugar levels before trying to conceive.
Read More: Diabetes quadruples birth defects risk, say researchers