William M. Callaghan, MD, MPH, FACOG

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Gestational diabetes mellitus (GDM), defined as carbohydrate intolerance leading to hyperglycemia with onset or first recognition during pregnancy, is among the most common complications of pregnancy. A recent US population-based analysis of hospital discharge data reported the overall prevalence of GDM for delivery hospitalizations as 4.2% for 2003-2004.

  However, prevalence rates for GDM vary by population studied and by criteria used to make the diagnosis.

  Moreover, administrative discharge data lack sensitivity for pregnancy-related conditions; therefore, the current estimated prevalence is likely an underestimate.

 There is evidence that GDM rates are increasing. The occurrence of GDM increased during the 1990s in northern California from 5.4% to 7.4% in just 6 years, leveling off at 6.9% in 2000.

  Similarly, GDM increased in Colorado from 2.1% to 4.1% between 1994 and 2002,

  and in a large group practice health maintenance organization in the Pacific Northwest, the rate of GDM increased from 2.9% in 1999 to 3.6% in 2006.

  Nationally, GDM rates, as reported on delivery discharge abstracts, increased 122% between 1989-1990 and 2003-2004.

 Normal pregnancy is characterized by increasing peripheral resistance to insulin and a compensatory increase in insulin secretion.

  Therefore, pregnancy might be viewed as a stress test for the glucose homeostasis mechanisms. That is, women who have some degree of chronic insulin resistance and compensatory increased insulin production resulting in beta-cell dysfunction before pregnancy may be unable to mount a sufficiently robust beta-cell response to pregnancy-mediated insulin resistance. 

In women with GDM, glucose homeostasis is sufficiently disturbed to result in consequences for both the mother and the infant. During pregnancy, these consequences are mostly related to excessive fetal growth and associated birth trauma, as well as increased operative delivery. After delivery, the overt glycemic abnormalities of GDM usually resolve, a phenomenon that suggests that GDM is transient and that the consequences of GDM end with the birth of the infant. However, for the woman, delivery is not the end of the story. The diagnosis of GDM heralds future health risks. Knowledge of this "failed" stress test conveys new information about the future risk for type 2 diabetes mellitus and informs screening and prevention efforts during the postpartum period and beyond. 

Why postpartum glucose screening is important

The majority of women with a GDM-affected pregnancy will have normal postpartum glucose testing, but they remain at risk for the development of type 2 diabetes. Although study results vary by the length of follow-up, the population screened, and the testing rate, an estimated 20% to 50% of women with GDM will develop type 2 diabetes within 5 to 10 years after delivery.

  These women would benefit, at a minimum, from periodic follow-ups to assess glucose status and lifestyle modifications. 

Either a fasting plasma glucose test or a 75-g, 2-hour oral glucose tolerance test can be used to diagnose diabetes at a postpartum visit. Although the fasting glucose test is easier to perform, the 2-hour test is more sensitive for diagnosing diabetes because it includes the possibility of a 2-hour value of 200 mg/dL or greater, even if the fasting level is normal (see Table).

  Moreover, the 75-g, 2-hour test is the only way to diagnose impaired glucose tolerance. However, neither test has been integrated well into contemporary clinical practice. Published screening rates range between approximately 30% and 55%, indicating that many women who had GDM are not screened postpartum.

In an effort to improve screening rates among woman who experience a GDM-affected pregnancy, a recent Committee Opinion from the American College of Obstetricians and Gynecologists' (ACOG) Committee on Obstetric Practice recommended that all women who have had a GDM-affected pregnancy be screened  6 to 12 weeks postpartum.

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Gestational diabetes mellitus defined as carbohydrate intolerance leading to hypoglycemia with onset or first recognition during pregnancy is one of the most common complications of pregnancy.

Author | William M. Callaghan, MD, MPH, FACOG

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