Is that your GP saying that or your diabetes specialist? Have you seen a gynaecologist to check that everything's ok?
There is brief info here:
http://www.diabetes.co.uk/pregnancy-complications/infertility-in-women.html
For any woman - diabetic or not - being in good general health is beneficial eg healthy weight, nutritious diet, etc.
If you haven't had any infertility investigations, I'd push for those. In the meantime, ask your DSN for advice about getting your HbA1C down. It can only help - now and in any future pregnancy