Gestational Diabetes: Worth Testing For? Worth Treating?

Is there evidence that treating elevated fasting glucose levels or impairedglucose tolerance, from early in pregnancy until term, improves outcomes(ie, fewer cesarean deliveries, fewer macrosomic babies, fewer patients in whompreeclampsia develops, fewer mothers who subsequently require insulin)?-- Richard S. Banfield, Jr, MD
   New Canaan, Conn
It has been said that "gestational diabetes is a diagnosisstill looking for a disease"; many experts, especiallyin the United Kingdom, have expressed doubts aboutthe existence of this entity. The Cochrane Databaseof Systematic Reviews reported no usefulness in thedetection and treatment of gestational diabetes, andconcluded that screening for this condition should bestopped. Both in 1996 and in 2003, the US PreventiveServices Task Force similarly concluded that there isinsufficient evidence to categorically support screeningfor gestational diabetes.¹ At the heart of this issue isthe clinical significance of gestational diabetes.Observational studies in the 1970s^2,3 suggested thatidentification of and intervention in gestational diabetesreduced perinatal morbidity (fetal macrosomia; birthtrauma; neonatal hypoglycemia, hypocalcemia, hyperbilirubinemia,and polycythemia). In patients with gestationaldiabetes treated with insulin because of fastinghyperglycemia, there is also an increased risk of intrauterinefetal death; this has been used to justify antepartumtesting and sometimes earlier delivery.More recent studies provide conflicting results.Rust and colleagues⁴ reported in 1996 that there was nodifference in the incidence of macrosomia between patientswith gestational diabetes and euglycemic controls.Casey and colleagues⁵ found a 12% risk of macrosomiain neonates of patients with class A1 diabetes; this wasnot significantly different from the 10% risk seen in thegeneral obstetric population. However, when womenwith class A1 gestational diabetes were compared withnondiabetic pregnant women matched for age, race,presence of obesity, and parity, the difference was significant(23% risk of macrosomia vs 12%, respectively).This indicates that the rate of macrosomia may be reducedby diagnosis and intervention in patients withclass A1 gestational diabetes. The Toronto TrihospitalGestational Diabetes Project⁶ assessed 145 pregnanciesin which gestational diabetes was identified: the studydemonstrated an increase in the rate of adverse fetaland maternal outcomes compared with that seen ineuglycemic controls; however, after adjustment for otherrisk factors, the independent effect of carbohydrate intoleranceon the outcomes was modest.Maternal morbidity associated with gestational diabetesincludes:

• A small increase in the relative risk of preeclampsia.
• Increased incidence of cesarean delivery, mainly consequentto fetal macrosomia.
• Increased incidence of traumatic vaginal delivery, alsomainly as a result of macrosomia.

Hence, gestational diabetes appears to affect primarilythe "quality of delivery." However, in the absenceof results from randomized clinical trials, the benefit ofscreening and subsequent interventions cannot be ascertained.In pregnant women with insulin-treated diabetes,the rate of fetal macrosomia is still 20% to 40%--even in those with good glycemic control.

⁷

In conclusion, there is insufficient evidence to supportthe current practice of screening for gestationaldiabetes. At the same time, there is not enough evidenceto recommend the discontinuation of this practice,which, at least in the United States, is consideredthe standard of care. One randomized clinical trial isunder way, conducted by the US National Institute ofChild Health and Human Development, Maternal-FetalMedicine Units Network; it involves approximately2400 women with mild gestational diabetes. Until theresults of this trial--and hopefully others--becomeavailable, the logical approach for the clinician is prudenceand a refraining from extremes.

-- Alex C. Vidaeff, MD
   Associate Professor of Obstetrics & Gynecology
   Maternal-Fetal Medicine Division
   University of Texas, Houston Medical School

References:

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2.

O’Sullivan JB, Charles D, Mahan CM, Dandrow RV. Gestational diabetes andperinatal mortality rate.

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3.

Gabbe SG, Mestman JG, Freeman RK, et al. Management and outcome ofclass A diabetes mellitus.

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4.

Rust OA, Bofill JA, Andrew ME, et al. Lowering the threshold for the diagnosisof gestational diabetes.

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Casey BM, Lucas MJ, Mcintire DD, Leveno KJ. Pregnancy outcomes inwomen with gestational diabetes compared with the general obstetric population.

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1997;90:869-873.

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Sermer M, Naylor CD, Farine D, et al, for the Toronto Tri-Hospital GestationalDiabetes Investigators. The Toronto Tri-Hospital Gestational Diabetes Project:a preliminary review.

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1998;21:B33-B42.

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Fraser R. Diabetic control in pregnancy and intrauterine growth of the fetus.

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