Having type-1 diabetes creates some challenges to having a healthy baby, but it certainly does not mean that your baby is destined to develop birth defects.
During the first 8-12 weeks of gestation, most of the baby's vital organs develop -- brain/nervous system, heart/lungs, and so on. It is during this phase that uncontrolled blood sugar levels can have a particularly harmful affect. Research has shown that keeping the A1c at around 7% or less during this phase (including the time of conception) greatly reduces the risk of major developmental defects and spontaneous abortion.
During the 2nd and 3rd trimesters (weeks 12 onward), tight glucose control is necessary to regulate the baby's rate of growth. "Feeding" the baby too much sugar from your bloodstream will cause him/her to grow too rapidly, and can lead to premature birth and complications during the birth itself.
While you cannot change the past, there is no time like the present to manage your diabetes as well as possible. Work with your diabetes doctor and a high-risk obstetrician on fine-tuning your insulin doses. You should also be screened for eye, kidney and blood pressure problems now, as they can become much worse during pregnancy.
I also recommend the book "Balancing Pregnancy With Pre-Existing Diabetes." It contains a great deal of helpful insight.