Importance: Pregestational diabetes refers to diabetes diagnosed before pregnancy. Gestational diabetes refers to any degree of glucose intolerance with onset or first recognition during pregnancy. Pregnant women with pregestational diabetes are at increased risk for multiple complications affecting both the mother and the fetus. The degree to which pregnant women with gestational diabetes are at increased risk for maternal or fetal complications is less certain.

Detection: Several different methods are used to screen for GDM; many women with positive screening test results do not meet current diagnostic criteria for GDM.

Benefits of Detection and Early Treatment:

• Screening before 24 weeks gestation: The evidence is poor to determine whether there are benefits to screening women at this time in pregnancy.
• Screening after 24 weeks gestation: Although screening and early treatment of GDM reduces macrosomia, and although 1 trial suggests the possibility of other health benefits, the overall evidence is poor to determine whether maternal or fetal complications are reduced by screening.

Harms of Detection and Early Treatment: There is fair evidence that short-term anxiety occurs in some women with positive screening results; longer term psychological or other harms have not been documented. The majority of positive screening test results are probably false positives. Consequently, it is likely that many women and medical practices are being inconvenienced unnecessarily by screening.

USPSTF Assessment: The USPSTF concludes that the current evidence is insufficient to assess the balance between the benefits and harms of screening women for GDM either before or after 24 weeks gestation.

Patient Population Under Consideration: This recommendation concerns pregnant women who have not previously been diagnosed with diabetes.

Suggestions for Practice Regarding the I Statement: Until there is better evidence, clinicians should discuss screening for GDM with their patients and make case-by-case decisions. Discussions should include information about the uncertainty of benefits and harms as well as the frequency of positive screening test results.

Assessment of Risk: Women who are obese, older than 25 years of age, have a family history of diabetes, have a history of previous GDM, or are of certain ethnic groups (Hispanic, American Indian, Asian, or African-American) are at increased risk of developing GDM.

Screening Tests: In the United States, the most common screening test is an initial 50-gram 1-hour glucose challenge test (GCT). If the result of the GCT is abnormal, variably defined as either greater than 130 mg/dL or 140 mg/dL, the patient undergoes a 100-gram 3-hour oral glucose tolerance test (OGTT). Two or more abnormal values on the OGTT are considered a diagnosis of GDM.

Time of Screening: Most screening is conducted between 24 and 28 weeks gestation. There is little evidence about the value of earlier screening.

Treatment: Treatment usually includes recommendations for physical activity and dietary modification. In addition, treatment sometimes includes medication (either insulin or oral hypoglycemic agents), support from diabetes educators and nutritionists, and increased surveillance in prenatal care. The extent to which these interventions improve health outcomes is uncertain.

Other Approaches to Prevention: Nearly all pregnant women should be encouraged to achieve moderate weight gain based on their pre-pregnancy body mass index (BMI) and to participate in physical activity.

Research Needs and Gaps: Prospective studies on the health outcomes of women with various glucose levels, adjusted for obesity, would help us better understand what level of glucose constitutes an important risk to mother or fetus. It is hoped that the Hyperglycemia and Adverse Pregnancy Outcome (HAPO) study, which is now in the data analysis phase, will provide useful information on this issue. Future studies should examine glucose levels before 24 weeks gestation as well as during the 24 to 28 week gestational period. They should present data on women with specific risk factors such as prepregnancy obesity, older- or younger-than-optimal age, and history of previous large-for-gestational-age births; along with data on women with no recognized risk factors. In addition, further randomized trials comparing the health outcomes of glucose lowering with the health outcomes of not intervening, for women who have screening-detected GDM, would add weight to the findings of the Australian Carbohydrate Intolerance in Pregnancy Study (ACHOIS). The Maternal-Fetal Medicine Unit network (MFMU) randomized, controlled trial now in progress,and intended to complete recruitment in the next 2 years, should hopefully help provide this information. In addition to outcomes studies, more definitive data are required on the most appropriate screening strategies for GDM, including information on the best glucose load and timing.

Burden of Disease: Gestational diabetes mellitus (GDM) is currently defined as any degree of glucose intolerance with onset or first recognition during pregnancy.¹ This definition does not exclude glucose intolerance that may have antedated pregnancy. The current prevalence of GDM in the United States ranges from 1% to 9%, depending on the characteristics of the population screened.^2-3 In women with defined low-risk factors, such as white ethnic origin, age younger than 25 years, and a body mass index (BMI) of less than 25 kg/m2, prevalence of GDM ranges from 1.4% to 2.8%.^4-9 The prevalence in women with defined high-risk factors, such as age older than 25 years, obesity, or a family history of diabetes, ranges from 3.3% to 6.1%.⁷ Higher rates have been reported in certain ethnic groups.^2-3 Variations in screening practices and in the prevalence of other risk factors make it difficult to quantify the independent contribution of race and ethnicity to the development of GDM.

Scope of Review: In 2003, the USPSTF concluded that the scientific evidence was insufficient to support a recommendation for or against routine screening of GDM in all pregnant women. There was fair-to-good evidence that screening, combined with therapy for GDM, can reduce the rate of fetal macrosomia, but the USPSTF was unable to find sufficient evidence that GDM screening reduced adverse health outcomes for mothers or their infants.¹⁰

With the increasing prevalence of U.S. women at high risk for type 2 diabetes and GDM, the issue of early screening is becoming increasingly important. The previous USPSTF review did not include evidence related to GDM screening before 24 weeks gestation. The current review considered all evidence from the previous review, and identified and evaluated evidence that has become available since the prior review, on the risks and benefits of GDM screening at 24 weeks or later. In addition, the USPSTF reviewed all available evidence pertaining to GDM screening before 24 weeks.

The USPSTF reviewed the evidence for benefits of screening in the following health outcomes: perinatal mortality, brachial plexus injury, clavicular fracture, maternal mortality, preeclampsia, and admission to neonatal intensive care units for hypoglycemia, hyperbilirubinemia, and respiratory distress syndrome. Intermediate outcomes, such as macrosomia and cesarean or vaginal delivery, were not systematically reviewed.

Accuracy of Screening Tests: There are studies that report the sensitivity and specificity of various GDM screening tests as predictors for GDM. However the USPSTF found no studies that met its inclusion criteria. The evaluation of screening test performance in GDM is complicated by the many different accepted standards for screening tests, diagnostic tests, and diagnostic criteria. Test performance can be evaluated only in the context of how accurately the test identifies people with disease (sensitivity) and excludes those without disease (specificity). However, with GDM, the "disease" is actually many potential outcomes — and for 2 different people (mother and baby). Additionally, the generally accepted primary outcomes (for example, stillbirth, neonatal death, brachial plexus injury) are rare events that make estimates unstable except in a very large study. Data to support specific timing for screening are also sparse.

Effectiveness of Early Detection and/or Treatment: No properly conducted randomized, controlled trial (RCT) has examined the benefit of universal or selective screening for GDM, compared with no screening. Two RCTs have been published that studied treatment versus no treatment of GDM in screening-detected populations – one recent (ACHOIS)¹¹ and one conducted more than 4 decades ago (a study by O'Sullivan and colleagues.¹² Both of these trials randomly assigned participants to treatment or no treatment of GDM on the basis of a universal screening program approach. ACHOIS reported that dietary management, glucose monitoring, and insulin treatment as needed, in 1,000 women with mild GDM diagnosed after 24 weeks gestation, improved the composite neonatal outcome, compared with no treatment.¹¹ The composite outcome was defined as one or more of the following: death, shoulder dystocia, bone fracture, and nerve palsy. The majority of the actual outcomes summed in this composite outcome were shoulder dystocia, an outcome not considered by the USPSTF review as a final health outcome. Perinatal mortality, although rare, did not occur in any of the 490 babies born to mothers who were treated, compared with 5 total stillbirths or neonatal deaths among the 510 women in the non-treated group. Women in the treatment group had a 30% percent lower risk for pregnancy-induced hypertension (defined as blood pressure > 140/90 mm Hg on 2 occasions more than 4 hours apart), compared with women who were not treated for GDM; the rate of pregnancy-induced hypertension was 18% in the untreated women and 12% in the treated group (adjusted relative risk, 0.70 [95% CI 0.51-0.95]). As glucose control was not part of data collection and was not reported, the relative impact of glycemic control (versus weight control) on the improvement of outcomes with treatment cannot be estimated. All that can be concluded is that treatment improved some outcomes.

The fair-quality RCT by O'Sullivan and colleagues found that treatment in a screened population of women at high risk for GDM (gestational age at screening unspecified) reduced the intermediate outcome of macrosomia, but did not reduce the perinatal mortality rate.¹² High risk was defined as a history of: delivery of a baby weighing more than 9 pounds, "toxemia" in 2 or more pregnancies, fetal or neonatal death, congenital anomaly, or prematurity. Treatment was a small daily dose of insulin (10 units) initially, with irregular glucose monitoring of urine and blood (glucose monitoring was not available 40 years ago). In contrast, the ACHOIS participants used insulin only if other therapies failed to achieve tight glycemic control based on the study's glucose targets, and only 17% of the treatment group required insulin.

The USPSTF identified no RCTs for screening and treatment before 24 weeks gestation. One fair-quality prospective cohort study of early screening and treatment for GDM was identified, and its results suggest that an early diagnosis of GDM may represent pregestational diabetes, because women diagnosed early (by an abnormal 50g GCT at the first prenatal visit) were more likely to require insulin, and had a higher proportion of hypertension, perinatal deaths, and neonatal hypoglycemia than those diagnosed at 24 to 28 weeks gestation.¹³

Potential Harms of Screening and/or Treatment: There are 2 potential domains of harms of screening and treatment for GDM: the psychological and the physical. The primary adverse effects associated with screening would be the psychological impact of screening on the mother with GDM, and potentially on the mother who does not have GDM but has the added time, cost, physical discomfort and psychological burden of screening and confirmatory diagnostic testing. A review of the literature revealed mixed available evidence on the initial psychological impact of GDM screening. In the first few weeks after screening, women who screened positive for GDM may report higher anxiety, more psychological distress, and poorer perceptions of their general health than women who screened negative. Available evidence, however, suggests that these differences, even if present shortly after diagnosis, do not persist into the late third trimester or postpartum period.^14-16

Further, ACHOIS found, in a subgroup that responded to the questionnaire, that treatment was potentially associated with overall improved self-reported health status and reduced post-partum depression at 3 months postpartum compared with no treatment.¹¹ Alternative explanations for the reduced postpartum depression and improved quality-of-life responses in the treated group could include unblinding prior to the 3-months postpartum period before the questionnaire was completed or what is sometimes termed the Hawthorne effect, in which the additional attention given to the treatment group, rather than the treatment itself, could improve perceptions. Finally, a prospective study found that mood did not differ in women treated for GDM compared with controls.¹⁷

For the mother, hypoglycemia is the potentially most serious physical harm. Not all studies monitored or reported maternal hypoglycemia, but in those that did the rates are low with treatment and no worse with alternate therapies.

With regard to potential fetal or newborn risks, the potential teratogenicity of certain newer treatments for GDM (oral hypoglycemic agents or insulin analogues) presents a potential physical harm to the fetus that clearly could relate to GDM treatment; however, most treatments for GDM start in the second trimester, after the period of major organogenesis. Thus, data are very limited to assess potential teratogenicity of newer agents for treatment.

One potential issue is the number of false-positive screening test results. Given the lack of evidence to determine the accuracy of screening tests, it is difficult to estimate how often this occurs. However, studies show that fewer than 1 in 5 women with a positive GCT will meet criteria for GDM on a full OGTT.¹³ This result indicates that many women are being inconvenienced, that health care services are being used unnecessarily, and that time is wasted evaluating false-positive test results.

Estimate of the Magnitude of Net Benefit: The USPSTF was unable to estimate the magnitude of net benefit, or indeed the existence of a benefit, of screening or treatment for GDM. This was due to a lack of studies of screening with a sufficient number of participants to permit evaluation of important health outcomes such as mortality, brachial plexus injury, clavicular fracture, and admission to neonatal intensive care units for hypoglycemia, hyperbilirubinemia, or respiratory distress syndrome. In addition, because of the lack of an accepted gold standard for screening, there is limited evidence on the accuracy of available screening strategies. There is also insufficient evidence on the benefits of treating GDM in improving health outcomes.

How Does Evidence Fit with Biological Understanding? Data on the overall impact of GDM screening and treatment are limited because most babies with macrosomia are born to mothers without GDM, and most cases of injuries related to shoulder dystocia occur in pregnancies with infants of normal birth-weight.¹⁸ The relationship between GDM and adverse outcomes is further confounded by the fact that maternal obesity is an independent risk factor for many of the same outcomes.¹⁸

The American College of Obstetricians and Gynecologists (ACOG) recommends that all pregnant women be screened for GDM by patient history, clinical risk factors, or a laboratory screening test.¹⁹ ACOG recognizes that low-risk women may be less likely to benefit from screening with laboratory testing; similarly, the American Diabetes Association (ADA) states that low-risk women need not be screened with glucose testing.^19-20 ACOG and ADA consider a woman to be at low risk for GDM if she meets all of the following criteria: (1) age younger than 25 years; (2) no membership in an ethnic group with increased risk for developing type 2 diabetes; (3) body mass index of 25 kg/m2 or less; (4) no previous history of abnormal glucose tolerance or adverse obstetrics outcomes usually associated with GDM; and (5) no known history of diabetes in a first-degree relative. The American Academy of Family Physicians has concluded that the evidence is insufficient to recommend for or against routine screening for gestational diabetes in asymptomatic pregnant women.²¹

Corresponding Author: Ned Calonge, MD, MPH, Chair, U.S. Preventive Services Task Force, c/o Program Director, 540 Gaither Road, Rockville, MD 20850.

Members of the U.S. Preventive Services Task Force*: Ned Calonge, MD, MPH, Chair, USPSTF (Colorado Department of Public Health and Environment, Denver, Colorado); Diana B. Petitti, MD, MPH , Vice-chair, USPSTF (Keck School of Medicine, University of Southern California, Sierra Madre, California); Thomas G. DeWitt, MD (Children's Hospital Medical Center, Cincinnati, Ohio); Leon Gordis, MD, MPH, DrPH (Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland); Kimberly D. Gregory, MD, MPH (Cedars-Sinai Medical Center, Los Angeles, California); Russell Harris, MD, MPH (University of North Carolina School of Medicine, Chapel Hill, North Carolina); George Isham, MD, MS, (HealthPartners, Minneapolis, Minnesota); Michael L. LeFevre, MD, MSPH (University of Missouri School of Medicine, Columbia, Missouri); Carol Loveland-Cherry, PhD, RN (University of Michigan School of Nursing, Ann Arbor, Michigan); Lucy N. Marion, PhD, RN (School of Nursing, Medical College of Georgia, Augusta, Georgia); Virginia A. Moyer, MD, MPH (University of Texas Health Science Center, Houston, Texas); Judith K. Ockene, PhD (University of Massachusetts Medical School, Worcester, Massachusetts); George F. Sawaya, MD (University of California, San Francisco, California); Albert Siu, MD (Mount Sinai School of Medicine, New York, New York; Steven M. Teutsch, MD, MPH (Merck & Company, Inc., West Point, Pennsylvania); and Barbara P. Yawn, MD, MSPH, MSc (Olmsted Medical Center, Rochester, MN).

*Members of the Task Force at the time this recommendation was finalized. For a list of current Task Force members, go to https://www.uspreventiveservicestaskforce.org/Page/Name/out-members.

This document is in the public domain within the United States.

Requests for linking or to incorporate content in electronic resources should be sent via the USPSTF contact form.

Source: U.S. Preventive Services Task Force. Screening for Gestational Diabetes Mellitus: U.S. Preventive Services Task Force Recommendation Statement. Ann Intern Med 2008;148:759-765.

AHRQ Publication No. 08-05115-EF-2.