Diabetes in pregnancy is associated with higher rates of miscarriage, pre-eclampsia, preterm labor, and fetal malformation. To prevent these obstetric and perinatal complications, women with diabetes have to control levels of blood sugar, both prior to and during pregnancy. Thus, individualized medical nutrition therapy for each stage of pregnancy is essential. We provided in-depth medical nutrition therapy to a 38-year-old pregnant woman with diabetes at all stages of pregnancy up to delivery. She underwent radiation therapy after surgery for breast cancer and was diagnosed with diabetes. At the time of diagnosis, her glycated hemoglobin level was 8.3% and she was planning her pregnancy. She started taking an oral hypoglycemic agent and received education regarding the management of diabetes and preconception care. She became pregnant while maintaining a glycated hemoglobin level of less than 6%. We provided education program for diabetes management during the pregnancy, together with insulin therapy. She experienced weight loss and ketones were detected; furthermore, she was taking in less than the recommended amount of foods for the regulation of blood sugar levels. By giving emotional support, we continued the counseling and achieved not only glycemic control but also instilled an appreciation of the importance of appropriate weight gain and coping with difficulties. Through careful diabetes management, the woman had a successful outcome for her pregnancy, other than entering preterm labor at 34 weeks. This study implicated that the important things in medical nutrition therapy for pregnant women with diabetes are frequent follow-up care and emotional approach through the pregnancy process.