Based upon the physician just saying to bill with it, even though it's a part of the patient's record, the physician who delivered needs to provide documentation that this affected, or was a co-morbidity to, possible outcomes of the delivery.
ICD-9 2008 Coding Guidelines advise that, "Diabetes mellitus is a significant complication factor in pregnancy. Pregnant women who are diabetic should be assigned code 648.0X, Diabetes mellitus commplicating pregnancy, and a secondary code from category 250.0X, Diabetes mellitus, to identify the type of diabetes. Code V58.67, Long-term (current) use of insulin should also be assigned if the diabetes mellitus is being treated with insulin."
Gestational diabetes is different with different codes. ICD-9 Coding Guidelines does also offer guidance on that.
With that being said, and this is just my two cents, I'd ask the delivering physician to addendum the delivery summary to include the diabetes condition as a complicating factor, as well as whether it was gestational or not to assist you with coding appropriately. From a legal standpoint, each chart note must stand alone. If the delivery summary were to be submitted to a carrier, or be reviewed by an attorney at OIG, there would be nothing to support DM at all.
Hope that helps.