There is no single cause for gestational diabetes. Scientists believe there may be several factors that contribute to high glucose (blood sugar) in pregnant women. The most common theory involves the production of hormones and their effect on insulin.

During pregnancy, the fetus is supported by the placenta, an organ that supplies the fetus with nutrients. The placenta produces hormones, such as estrogen, that help the baby develop. These hormones may also interfere with the mother's insulin in her body. The hormones appear to block cells, making it harder for the glucose to reach the body. This condition, known as insulin resistance, contributes to the cause of gestational diabetes.

New research suggests that a protein called menin, which limits growth of pancreatic cells, may play a role in the development of gestational diabetes.

Insulin resistance usually begins midway in the pregnancy, around 20 to 24 weeks. As the placenta grows, it produces more hormones, which cause greater insulin resistance.

In most pregnant women, the pancreas is able to produce more insulin, about three times the normal amount. This increased insulin is necessary to overcome the insulin resistance caused by the higher level of hormones. In some women, however, the pancreas is unable to make enough insulin. When the pancreas produces as much insulin as possible and it is still not enough to overcome the effect of the placenta's hormones, gestational diabetes is the result.

Any pregnant woman can develop gestational diabetes, but certain women are at higher risk. Factors that increase the chance of developing the condition include:

• Being obese or overweight before becoming pregnant
  
  
• Being a member of certain races or ethnic groups (e.g., black, Hispanic, Native American, Asian, Pacific Islander)
  
  
• Being over age 25
  
  
• Family history of diabetes
  
  
• Having prediabetes or insulin resistance
  
  
• Having gestational diabetes in a previous pregnancy
  
  
• Previously giving birth to a stillborn baby or baby over 9 pounds (macrosomia)
  
  
• Having too much amniotic fluid (polyhydramnios)
  
  
• Having glucose in the urine (glucosuria)
  
  
• Polycystic ovarian syndrome
  
  
• Use of certain contraceptives (progestin-only "mini-pill" oral contraceptives and, possibly, injections of depot medroxyprogesterone, DMPA)

Genetics appears to play several roles in the development of gestational diabetes. Scientists have found that a certain variant of a gene called the calpain-10 gene leads to gestational diabetes. In addition, several studies have suggested that short women may be more likely to develop gestational diabetes.