8th November 2018, Dr Chee L Khoo
When there is so much talk about gestational diabetes (GDM), you can’t help but think that just about every pregnant woman has GDM in south west Sydney (SWS). After all, if you look at the “high risk” group that you ought to be screening for GDM, there is hardly any woman in SWS that is not in the high risk category. It then follows that you would screen everyone with an oral glucose tolerance tests (OGTT) when they are first diagnosed with pregnancy. I mean, after all, where is the harm? Is there? Are we over diagnosing GDM? Are we over treating GDM?
Before we can answer the questions above, we need look back at how we got here in the first place. O’ Sullivan and Mahan proposed the first diagnostic criteria for GDM in 1964 (1). It was based on a 3 hour oral OGTT. Over the subsequent 25 years, there were many changes to the cut off glucose levels (National Diabetes Data Group 1979 (2) and Carpenter and Coustan 1982 (3) for the diagnosis of GDM. All the diagnostic thresholds were based on data from women who were diagnosed with diabetes after gestation and not on any short-term adverse pregnancy outcomes. The criteria for the diagnosis of GDM were based on the future risk of the mother developing diabetes.
In 2010, the International Association of Diabetes and Pregnancy Groups (IADPSG) proposed a new set of criteria, based on the incidence of adverse perinatal outcomes, as assessed in the Hyperglycemia and Adverse Pregnancy Outcomes (HAPO) study (4). We long suspected that high maternal glucose wasn’t a good thing for the foetus but the HAPO provided the evidence of the harm. It also showed that the higher the glucose levels were, the higher the likelihood of adverse outcomes.
The IADPSG recommended the 75g 2-hour OGTT diagnostic criteria based on the HAPO study’s in 2010. The screening and diagnostic methods of GDM and the diagnostic criteria differ among various associations and organisations. Further, who to screen and when to screen was also different in different jurisdictions.
There is no question that hyperglycaemia is associated with adverse pregnancy outcomes for both mother and baby. Initial studies disputed the benefits of GDM treatment on foetal and maternal outcomes (Cochrane Collaboration systematic review 2003 (5), Garner et al 1997 (6), Bankroft 2000 (7)) but subsequent and more recent studies demonstrated reduction in adverse pregnancy outcomes when GDM is treated (ACHOIS 2005 (8), Langer et al 2005 (9))
There are many reasons for the inconsistency in the results – different screening tests used, different population screened and different diagnostic criteria used. There is more to this uncertainty, though.
Early diagnosis of GDM
The original screening for GDM was based on OGTT performed at 24-28 weeks gestation. Evidence of the benefits of treating GDM were based on detecting hyperglycaemia at 24-28 weeks gestation (10). In women in high risk groups, an OGTT is often now performed early in pregnancy. If you think there is much debate on the diagnostic criteria for GDM when the OGTT is performed at 24-28 weeks, the diagnostic criteria for testing in early pregnancy is even more uncertain.
Some of the women who were diagnosed with GDM in early pregnancy may not have hyperglycaemia when tested later at 24-28 weeks. We do have evidence that women with hyperglycaemia under 24 weeks have significantly increased risk for perinatal mortality, neonatal hypoglycemia, and insulin therapy (11). However, this group of women are not a homogenous group. There are some who has elements of metabolic syndrome present. There are some who has evidence of autoimmunity while others have impaired beta cells. Their glucose profile during pregnancy may also vary.
Treatment of early GDM?
In addition to the lack of data to inform us as to what the diagnostic criteria for early GDM is and who we should really screen, we also lack evidence whether early treatment will prevent adverse pregnancy outcomes. There are concerns about risks from over-treatment. Women who have undergone bariatric surgery have increased rates of small for gestational age (SGA). We have similar reports in obese women who have lost weight during pregnancy. If the foetus is exposed to relative undernutrition from over treatment, foetal programming for future metabolic disease could increase.
Because of the gap in our knowledge, SWS has now revised their GDM screening protocol.
Click here for risk factors for GDM
Click here for GDM diagnostic criteria for SWS
Click here for Screening Protocol for GDM
As part of an NHMRC multi-centre funded study, SWS are undertaking a study to see if treatment is really needed in women with mild hyperglycaemia in early pregnancy. SWS is leading the way to fill in the knowledge gap. You may have women who qualify for the study. Click here to see the inclusion criteria for the TOBOGM study. Do not send them for an OGTT.
- OSullivan JB, Mahan CM. Criteria for the Oral Glucose Tolerance Test in Pregnancy. 1964;13:278–285
- National Diabetes Data Group. Classification and diagnosis of diabetes mellitus and other categories of glucose intolerance. 1979;28:1039–1057.
- Carpenter MW, Coustan DR. Criteria for screening tests for gestational diabetes. Am J Obstet 1982;144:768–773
- HAPO Study Cooperative Research Group,Metzger BE, Lowe LP, Dyer AR, Trimble ER, Chaovarindr U, et al. Hyperglycemia and adverse pregnancy outcomes. N Engl J Med. 2008;358(19):1991–
- Tuffnell DJ, West J, Walkinshaw SA. Treatments for gestational diabetes and impaired glucose tolerance in pregnancy. Cochrane Database Syst Rev. 2003 CD003395
- Garner P, Okun N, Keely E, Wells G, Perkins S, Sylvain J, et al. A randomized controlled trial of strict glycemic control and tertiary level obstetric care versus routine obstetric care in the management of gestational diabetes: a pilot study. Am J Obstet Gynecol. 1997;177:190–195
- Bancroft K, Tuffnell DJ, Mason GC, Rogerson LJ, Mansfield M. A randomised controlled pilot study of the management of gestational impaired glucose tolerance. 2000;107:959–963
- Crowther CA, Hiller JE, Moss JR, McPhee AJ, Jeffries WS, Robinson JS. Effect of treatment of gestational diabetes mellitus on pregnancy outcomes. N Engl J Med. 2005;352:2477–2486
- Langer O, Yogev Y, Most O, Xenakis EM. Gestational diabetes: the consequences of not treating. Am J Obstet Gynecol. 2005;192:989–997.
- Landon MB, Spong CY, Thom E, Carpenter MW, Ramin SM, Casey B, et al. A multicenter, randomized trial of treatment for mild gestational diabetes. N Eng. l J Med. 2009;361:1339–1348.
- Jincy Immanuel, David Simmons. Screening and Treatment for Early-Onset Gestational Diabetes Mellitus: a Systematic Review and Meta-analysis. Curr Diab Rep (2017) 17: 115