The exact cause of gestational diabetes in not clear and there is several theories which try to explain the cause of gestational diabetes, but gestational diabetes is not due to lack of insulin as is the case in type-2 diabetes, although the pathophysiologically is similar to type II diabetes.

Insulin resistance increases during pregnancy, may be due to blocking effects of other hormones on the insulin. Due to increase insulin resistance the secretion of insulin is increased to maintain the metabolism of glucose. The pregnant woman’s hunger increased due to increased insulin, eating more as a result. Increased level of insulin causes reduction of number of insulin receptors which increases the insulin resistance further, thus creating a vicious cycle.

During pregnancy placenta supplies the growing fetus with all the nutrients including water. Placenta also produce a variety of hormones to maintain the pregnancy, such as estrogen, cortisol, and human placental lactogen which have a blocking effect on insulin and increase insulin resistance. This effect begins at about 20 to 24 weeks of pregnancy and called “contra-insulin effect”. As placenta grows in size during advancing of pregnancy the hormones secretion increase, which have blocking effect on insulin and increase insulin resistance further. To overcome insulin resistance the pancreas tries to compensate insulin resistance by increasing secretion of insulin. If the insulin supply by pancreas can not overcome the insulin resistance (insufficient to overcome the effect of the placental hormones), Gestational Diabetes results.

Certain pregnant women are at high risk of developing gestational diabetes and are screened for Gestational Diabetes using the following criteria:

• Obesity. Obese pregnant women are at higher risk of developing Gestational Diabetes.
• Age of pregnant woman more than 25 years.
• A family history of diabetes.
• Have polyhydramnios (excess amniotic fluid).
• History of previous pregnancy with complication such as big baby, still birth, or a child with a birth defect.
• History of glucose intolerance.
• If pregnant woman belong to a racial group with high prevalence of gestational diabetes.