What are the chances of someone with type 1 having a baby with anencephaly?

Slight increased: There is a slight increase in anencephaly with type 1 diabetes. The major factor for this and other neural tube defects is Folic Acid deficiency. Type 1 diabetes uncontrolled can also lead to a large number and variety of other birth defects.

Increased anomalies: Women with diabetes that is poorly controlled in the weeks prior to and the first weeks of pregnancy have a higher chance of having a fetus with abnormalities of the heart and other parts of the body. It is important that they see a doctor and improve glycemic control before even trying to conceive. Screening and ultrasound studies can detect many of these abnormalities, but prevention is key.

HbgA1C?: If your glycated hemoglobin is <6% i would not anticipate any significant increased risk for birth defects. If it is >10% then the risk is >10-15% for major/minor birth defects including anencephaly. Control you blood sugars for at least 3-6 months before getting pregnant and take Folic Acid daily.