Gestational Diabetes Mellitus

How to Cite This Chapter: Rodríguez-Gutiérrez R, Portillo-Sánchez P, Hinojosa-Amaya JM, Sieradzki J, Płaczkiewicz-Jankowska E. Gestational Diabetes Mellitus. McMaster Textbook of Internal Medicine. Kraków: Medycyna Praktyczna. Accessed July 15, 2020.
Last Updated: June 11, 2019
Last Reviewed: June 11, 2019
Chapter Information

Definition, Etiology, PathogenesisTop

Gestational diabetes mellitus (GDM) describes diabetes mellitus (DM) diagnosed in the second or third trimester of pregnancy in patients who meet ≥1 of the appropriate diagnostic criteria without clear diagnosis of overt diabetes prior to that time. Experts differ slightly in their views on optimal strategies for the diagnosis of GDM (Table 5.2-16).

Risk factors include multiparity, age >35 years, previous delivery of a child >4000 g of birth weight, delivery of a child with malformations, intrauterine death, hypertension or a body mass index (BMI) >30 kg/m2 before pregnancy, high-risk ethnic groups (eg, Hispanics, African American), family history of type 2 DM, or history of GDM (in ~30% of patients GDM develops again in a subsequent pregnancy).


The American Diabetes Association (ADA) suggests measuring fasting plasma glucose (FPG) levels and glycated hemoglobin (HbA1c) in every pregnant woman at the first office visit to diagnose previously unrecognized type 2 DM.

If FPG and HbA1c are within the reference range (<100 mg/dL and <6%, respectively) at the first visit, preferably in the first trimester, in all pregnant women within 24 to 28 weeks of pregnancy the ADA suggests to perform:

1) One-step diagnostic oral glucose tolerance test (OGTT) (fasting, with the administration of 75 g of glucose); or

2) Two-step OGTT, which starts with a nonfasting 50-g OGTT. If the result is ≥7.8 mmol/L (140 mg/dL) at 1 hour, then a fasting 100-g 3-hour OGTT is required.

GDM is diagnosed in patients with ≥1 abnormal glucose level found in the 1-step 75-g OGTT and 2 abnormal glucose levels in the 100-g OGTT (Table 5.2-16).

In patients in whom blood glucose levels fail to normalize after delivery (6-12 weeks), testing for DM using standard diabetes criteria is recommended (see Diabetes Mellitus).

Diagnostic Criteria

See Table 5.2-16.


1. Start with nutritional therapy. There is a paucity of evidence that one diet compared to others improves maternal or fetal outcomes.Evidence 1 Weak recommendation (downsides likely outweigh benefits, but the balance is close or uncertain; an alternative course of action may be better for some patients). Low Quality of Evidence (moderate confidence that we know true effects of the intervention). Quality of Evidence lowered due the risk of bias, imprecision, and indirectness to patient-important outcomes. Han S, Crowther CA, Middleton P, Heatley E. Different types of dietary advice for women with gestational diabetes mellitus. Cochrane Database Syst Rev. 2013 Mar 28;(3):CD009275. doi: 10.1002/14651858.CD009275.pub2. Review. Update in: Cochrane Database Syst Rev. 2017 Feb 25;2:CD009275. PubMed PMID: 23543574. However, ~80% of pregnant patients with GDM can be controlled solely with an adequate diet. The daily caloric intake depends on the prepregnancy BMI, physical activity, and term of pregnancy:

1) Underweight: 40 kcal/kg/d.

2) Normal weight: 30 kcal/kg/d.

3) Overweight and obesity: 20 to 25 kcal/kg/d.

4) Morbid obesity (BMI ≥40): 12 to 14 kcal/kg/d.

The suggested diet may include 35% to 45% of carbohydrates (mainly complex carbohydrates), 25% to 35% of fats (with equal proportions of saturated and unsaturated fats), and 20% of protein (1.3 g/kg/d). It is recommended to substitute a percentage of animal protein by nonanimal protein (eg, broccoli, mushrooms). A high fiber (≥20 g/1000 kcal) daily intake is suggested. It is also recommended to consume 3 moderate-sized meals in addition to 2 to 4 snacks, including an evening snack.

The Dietary Reference Intake (DRI) suggests a minimal consumption of 175 g of carbohydrate, 71 g of protein, and 24 g of fiber. The design of nutritional therapy has to be individualized in each patient and it is highly recommended that the assessment is done by a dietitian with expertise in GDM. The goal of this treatment is to have an adequate balance in caloric intake that benefits both the offspring and the mother and to achieve adequate glycemic control.

2. Pharmacologic therapy: In patients who have been compliant with appropriate nutritional therapy for 5 to 7 days and have not achieved normal blood glucose levels (criteria for glycemic control: see Diabetes Mellitus in Pregnancy) or those with an initial highly elevated glucose level, insulin is suggested as the first-line therapy. Intensive insulin therapy (a multiple daily injection regimen) using subcutaneous injections of short-acting human insulin or a rapid-acting insulin analogue and intermediate-acting human insulin (insulin isophane [NPH]) can be started. There is not enough data from randomized trials to prove the safety of long-acting insulin analogues (glargine and detemir) and NPH is preferred. Oral glucose-lowering drugs were previously considered to be contraindicated; however, metformin and glyburide (alone or in combination with insulin) have been proven effective and safe in patients with GDM and can be also used as a first-choice or add-on therapy. Metformin, compared with other therapeutic options like insulin or sulfonylureas, has been associated with smaller weight gain and lower prevalence of pregnancy-induced hypertension and neonatal hypoglycemia despite the fact that metformin crosses the placental barrier. Glyburide can also be used during pregnancy, but it has been associated with increased prevalence of neonatal hypoglycemia (fetal hyperinsulinism) and birth weights ≥4000 g.Evidence 2Weak recommendation (downsides likely outweigh benefits, but the balance is close or uncertain; an alternative course of action may be better for some patients). Moderate Quality of Evidence (moderate confidence that we know true effects of the intervention). Balsells M, García-Patterson A, Solà I, Roqué M, Gich I, Corcoy R. Glibenclamide, metformin, and insulin for the treatment of gestational diabetes: a systematic review and meta-analysis. BMJ. 2015 Jan 21;350:h102. doi: 10.1136/bmj.h102. Review. PubMed PMID: 25609400; PubMed Central PMCID: PMC4301599.

3. Management during delivery: In patients treated with insulin, the management is the same as in preexisting DM (see Diabetes Mellitus in Pregnancy). In women who have achieved satisfactory blood glucose control with diet alone, insulin should be administered during delivery only if blood glucose levels are >7.2 mmol/L (130 mg/dL).

4. Any type of therapy should be discontinued immediately after delivery unless there is a high suspicion for type 1 DM. Since patients with GDM are at increased risk of developing type 2 DM, a 2-hour 75-g OGTT is recommended 6 to 12 weeks after delivery. Patients meeting the criteria for prediabetes should be counseled about their increased risk of DM and should consider lifestyle modification strategies (nutritional therapy, metformin and exercise). In patients with a diagnosis of DM, start lifestyle changes and oral medications as in any other patient with a recent diagnosis of type 2 DM.


Table 5.2-16. Diagnostic criteria for gestational diabetes mellitus

75-g OGTT

100-g OGTT




O’Sullivan and Mahanb


Carpenter and Coustanb,c,d


Venous plasma

Venous plasma

Venous plasma

Venous whole blood

Venous plasma

Venous plasma

Fasting glucose levels

5.1-6.9 mmol/L

(92-125 mg/dL)

≥6.0 mmol/L

(108 mg/dL)

≥5.1 mmol/L

(92 mg/dL)

≥5.0 mmol/L

(90 mg/dL)

≥5.8 mmol/L

(105 mg/dL)

≥5.3 mmol/L

(95 mg/dL)

1 h

≥10.0 mmol/L

(180 mg/dL)

≥10.0 mmol/L

(180 mg/dL)

>9.1 mmol/L

(164 mg/dL)

≥10.6 mmol/L

(190 mg/dL)

≥10.0 mmol/L

(180 mg/dL)

2 h

8.5-11.0 mmol/L

(153-199 mg/dL)

≥9.0 mmol/L

(162 mg/dL)

≥8.5 mmol/L

(153 mg/dL)

>8.0 mmol/L

(144 mg/dL)

≥9.2 mmol/L

(165 mg/dL)

≥8.6 mmol/L

(155 mg/dL)

3 h

>6.9 mmol/L

(124 mg/dL)

≥8.0 mmol/L

(145 mg/dL)

≥7.8 mmol/L

(140 mg/dL)

a Diagnosis is defined by one abnormal glucose value at any time point.

b Diagnosis is defined by two or more abnormal values at different time points.

c Diagnostic thresholds recommended by the American College of Obstetricians and Gynecologists.

d Diagnostic thresholds recommended by the American Diabetes Association.

EASD, European Association for the Study of Diabetes; IADPSG, International Association of Diabetes and Pregnancy Study Group; NDDG, National Diabetes Data Group; OGTT, oral glucose tolerance test; WHO, World Health Organization.

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