Gestational diabetes mellitus (GDM) is common and often treated with insulin. Insulin allergy is rare and theoretically should be less common in pregnancy due to the immunosuppressed state. Two cases are presented of insulin allergy during GDM management. Case one is a 36 year old female (Gravida 3, Parity 2) of Serbian background, pre-pregnancy body mass index (BMI) 20.4 kg/m², diagnosed with GDM at 16 weeks gestation, on a background of previous insulin treated GDM during her second pregnancy in 2017. There was no family history of diabetes, nor issues with insulin therapy or foetal complications. Insulin isophane was commenced at 24 weeks gestation, with a starting dose of 6 units nocte. Insulin was gradually up-titrated to 24 units at 33 weeks gestation. At 36 weeks gestation, 12 weeks after commencement of insulin isophane, she developed a pruritic wheal rash at the sites of injection of insulin isophane, 30 minutes after injection and lasting 24 hours. Case 2 is a 31 year old female (Gravida 4, Parity 2) who was diagnosed with GDM at 16 weeks gestation. This was on a background of previous insulin treated GDM during her third pregnancy in 2014, during which there was a previous localised insulin allergy rash, and macrosomia (birth weight 4850g). There was a history of penicillin allergy rash, there was no family history of diabetes. Pre-pregnancy BMI was 40.2 kg/m². At 21 weeks gestation insulin isophane was commenced at a dose of 10 units nocte. A non-pruritic wheal rash developed at the sites of insulin isophane injection as soon as this was commenced. In both cases the symptoms were mild and the patients agreed to continue insulin isophane for the remainder of the pregnancy. Both cases represent IgE-mediated type I hypersensitivity reactions due to the characteristics and timing. Insulin allergies are rare in pregnancy but can occur.