Gestational Diabetes in Primary Care

Paul Hicks, MD

In This Article

Diabetes in Pregnancy

Diabetes mellitus (DM) can occur during pregnancy in 2 forms: pregestational and gestational diabetes. Pregestational diabetes is defined as Type I or Type II DM that existed before conception. Gestational diabetes (GDM) is defined as glucose intolerance that is first detected during the pregnancy and is associated with a probable resolution after the end of the pregnancy.[1]

Despite the defining feature of glucose intolerance, pregestational diabetes and GDM and are very different entities. Pregestational diabetes represents very high-risk obstetrics. Poor glucose control before conception and during organogenesis places the fetus at high risk of congenital malformations, especially cardiac and neural tube defects.[2] Women with pregestational diabetes have a higher risk of diabetic ketoacidosis and require careful and frequent monitoring to manage their complex insulin needs. Intensive fetal monitoring to identify and anticipate complications is also necessary.[3,4,5] Extensive experience and training are required to feel comfortable managing the care of women with pregestational diabetes. By contrast, if good glucose control can be achieved with diet (and insulin, if necessary), GDM confers a much lower risk for both the mother and fetus. The remainder of this article focuses on the woman with GDM.

Etiology of Gestational Diabetes

Pregnancy is a diabetogenic state. The hormones that lead to fetal growth and development do so by mobilizing the woman's nutritional resources, primarily glucose, and making them available to the fetus. Figure 1 illustrates the plasma levels of the critical anabolic hormones present during pregnancy. All increase dramatically in the last 20 weeks of gestation. Human placental lactogen plays a pivotal role in triggering the changes that can lead to glucose intolerance. It has strong anti-insulin and lipolytic effects. Peripheral insulin sensitivity during the third trimester decreases to 50% of that seen in the first trimester, and basal hepatic glucose output is 30% higher despite higher insulin levels.[6]

Figure 1.

Changes in plasma levels of hormones of pregnancy during normal gestation.

HCG = human chorionic gonadotropin; HCS (HPL) = human placental lactogen.
Reprinted with permission from Freinkel.[61]

This combination of increased mobilization of glucose, along with decreased sensitivity to insulin, places women at risk of developing diabetes during pregnancy; however, not all women do. There is evidence that women who develop GDM secrete less insulin in response to a glucose load than women who do not develop the disease.[7,8,9] Unfortunately, the reasons for this (and potential ways to prevent it) are not well understood.

Demographics of Gestational Diabetes

Approximately 4% of all pregnant women develop GDM, making it about 100 times more common than pregestational diabetes.[10] Certain subgroups of women are at much higher risk. At particularly high risk are the Pima Indians of Arizona, whose risk (approximately 40%) is 10-fold higher than that of the general population.[11] Other groups at increased risk are African Americans, obese women, women of advanced maternal age, those with a prior history of GDM or family history of DM, and women who have had previous large for gestational age (LGA) babies.[12,13] (The latter group possibly represents women in whom GDM was missed or who had minimally abnormal glucose tolerance tests). Nahum and Huffaker[14] examined the effects of race on prevalence of GDM and found that African American women have a 1.5-fold higher rate of GDM than white women and a 2-fold higher risk than Filipino women ( Table 1 ).

Other groups of women have a much lower risk of GDM. Teens have a relative risk approximately one fourth that of women aged 35 and older (OR 1.0 vs 4.2).[10] The relative risks of different ethnic groups have yet to be completely defined in teens. Nahum and Huffaker found a lower risk in Asian/Asian American and Filipino women; however, their results are inconsistent with those of other studies, and results by race/ethnicity must take into account diet and length of residence in the United States.


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