What is gestational diabetes mellitus (GDM) and who is at risk?
Gestational diabetes mellitus, GDM for short, is a condition in which some women develop high blood sugar levels (hyperglycemia) during pregnancy (usually in the second trimester). In most cases the condition returns to normal after delivery. This form of temporary diabetes affects about one in eight to ten women. It causes few symptoms but can be harmful to both the mother and the unborn child. Therefore, it is important that women are screened for the condition during pregnancy at the right time.

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The risk for GDM is higher in women who are over 30 years old, obese or overweight, have a family history of diabetes (first-degree relatives), or a history of diabetes in a previous pregnancy or problems in previous births, such as a large baby, stillbirth or repeated miscarriages, or in women belonging to ethnic groups with a high rate of diabetes such as Asians, Pacific Islanders, Latinos, etc. More than half of women who develop GDM may have no known risk factors. Therefore all pregnant women should be screened for GDM as advised by their doctor.

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How is GDM diagnosed?
Different protocols are used in different parts of the world to screen for and diagnose GDM. 
Criteria and blood glucose values for ruling out GDM vary slightly based on risk assessment, local conditions, and established practice.
The World Health Organization (WHO) recently introduced a new criterion based on 75 g (0.16 lb) oral glucose tolerance test (OGTT). For this test, pregnant women drink 75 g of glucose dissolved in 300 ml of water after an overnight fast. Blood samples are collected for glucose measurement just before, one and two hours after the glucose drink. GDM is diagnosed when test results indicate specific values. Ideally, all women not previously known to have diabetes should be screened for diabetes at the time pregnancy is first confirmed. This is done using any of the standard recommended tests for the diagnosis of diabetes. 
The tolerance test (OGTT) should be repeated between 24 and 28 weeks of pregnancy. High-risk women may be screened in each trimester of pregnancy.

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What causes GDM?
To maintain the pregnancy and ensure that the growing baby in the womb gets enough nourishment from the mother, the placenta (the organ that anchors the baby to the womb) produces hormones that counteract the effects of insulin (needed to use and energy storage) in your body.
The purpose of these insulin hormones is to raise your blood sugar and make nutrients available for transport to the baby. To balance this, your body produces more insulin, to prevent blood sugar from rising and to help store energy for later needs during pregnancy and breastfeeding. This results in the weight gain seen during pregnancy.
During the second half of pregnancy when the placenta is fully developed and the baby begins to grow, more nutrition is required, the level and effect of these insulin hormones are important. To deal with this, your pancreas has to produce more and more insulin. Some women (see risk factors listed above) are unable to increase their insulin production to overcome the effects of insulin hormones. In this situation, blood sugar begins to rise, resulting in gestational diabetes.
When the baby is born, the placenta is detached and pushed out of the uterus. Now there is nothing to oppose the effect of maternal insulin, so insulin needs are reduced. The insulin produced by your body is sufficient to keep your blood sugar under control and the diabetes goes away. Unfortunately women who develop GDM continue to be at increased risk of developing GDM in subsequent pregnancies. Also, if postpartum weight loss precautions and changes to a healthier lifestyle are not taken, 50% or more of women with GDM develop full-blown diabetes within ten years of GDM pregnancy.

What are the complications of GDM?
If GDM is diagnosed early and managed properly, chances are it will cause major problems
are low. However, if diagnosis is delayed or blood sugar levels are not kept under control with
proper treatment, the chances of complications affecting both mother and baby increase.
Pregnant women with diabetes are more likely to develop high blood pressure and preeclampsia.

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