What is Gestational Diabetes?
Gestational diabetes is a diagnosis of diabetes which occurs in the second and third trimester of pregnancy (1). It is characterised by the delayed response of the pancreatic beta cells to blood glucose and insulin resistance secondary to placental hormone release (1). Gestational diabetes does not include women with pre-existing Type 1 or Type 2 diabetes. There are many factors that can put women at greater risk of developing gestational diabetes including pre-existing insulin resistance, PCOS, obesity, cardiovascular disease and family history (1). Gestational diabetes is associated with a range of both fetal and maternal complications as well as an increased risk of developing type 2 diabetes later in life for both the mother and the fetus (1). Timely screening and detection can help to improve both maternal and fetal outcomes. In Australia, a screening test for gestational diabetes is provided for all women at around 24-28 weeks gestation.
Screening for gestational diabetes
Gestational diabetes is associated with both fetal and maternal complications. The consequences of poorly managed gestational diabetes in the foetus include macrosomia, neonatal hypoglycemia, polycythemia, shoulder dystocia, hyperbilirubinemia, neonatal respiratory distress syndrome increased perinatal mortality, and hypocalcemia (1). Maternal complications include hypertension, preeclampsia, increased risk of developing diabetes mellitus later in life, and increased risk of cesarean delivery (1). Gestational diabetes screening provides an opportunity to control blood glucose levels in pregnancy and therefore minimise the risk of pregnancy complications and adverse outcomes in the fetus (3).
In Australia all women undergo gestational diabetes screening between 24-28 weeks gestation (2). The test for gestational diabetes is called an oral glucose tolerance test (OGTT). The test works by assessing the pregnant woman’s response to glucose intake. Higher levels of glucose in the blood suggests that the body has had a hard time clearing glucose from the blood stream (1). In order to take the test the pregnant woman is advised to fast for at least 10-12 hours. A fasting glucose blood test is drawn and then the pregnant woman drinks a 75mg glucose drink. Two further blood tests are taken at one hour post consumption and two hours post consumption (2). To eliminate a gestational diabetes diagnosis fasting plasma glucose should be equal to or less than 5.1 mmol/L, 1 hour plasma glucose should be equal to or less than 10.0 mmol/L and 2 hour plasma glucose should be less than or equal to 8.5 mmol/L (2). Diagnoses is made when there is at least one elevated value (2). This test is called a “one step” glucose tolerance test which replaced the “two step” glucose tolerance in 2014 (6). The two step glucose tolerance test involved a second drink. Pregnant women who are considered high risk for gestational diabetes can be offered screening earlier. However early testing for blood glucose is currently not evidence based as elevated blood sugar in early pregnancy is not always predictive of a gestational diabetes diagnosis (2). As there is no conclusive evidence for glucose levels in early pregnancy, the Australian Journal of General Practitioners has provided the following measurements as a guide only: Fasting glucose between 5.1-5.9 mmol/L uncertain significance with recommendation to perform the OGTT at 16-18 weeks. Fasting glucose between 6.0-6.9 mmol/L more closely associated with subsequent gestational diabetes diagnosis, OGTT should also be performed at 16-18 weeks. A random plasma glucose of 9.0-11.0 mmol/L warrants further testing, usually with a fasting blood glucose test or HbA1c test (glycated haemoglobin). A HbA1c between 5.9-6.4% is reasonably predictive of gestational diabetes (2). HbA1c is gaining increased credibility for diabetes diagnosis and will likely play a larger role in future screening for gestational diabetes.
Concerns with screening:
In 2010, new guidelines were developed suggesting the blood glucose levels needed to obtain a gestational diabetes diagnosis should be lowered. This occurred following the large HAPO study which studied 23,000 women and found a worsening of maternal and foetal outcomes with blood sugar levels that were previously considered normal (5). While the guidelines didn’t come into full effect until 2014 in Australia, the lowered threshold along with an increase in maternal age and obesity caused a spike in the incidence of gestational diabetes diagnosis from 5-15% in just a few short years (6). Additionally the change in guidelines from the two step approach to the one-step approach resulted in an increase in gestational diabetes diagnosis without necessarily contributing to worsening of fetal and maternal outcomes (7). This has led to concerns of over diagnosis and over treatment. Theoretically a lowered diabetes diagnosis threshold would have the advantage of earlier intervention to stop the progression into more advanced gestational diabetes, however recent research does not support this idea for pregnant women with only mild elevations in blood glucose. The use of lower glycemic criteria has not shown to reduce the risk of large for gestational age babies but may increase the risk of hypoglycemia in the infant and potentially also increase small for gestational age babies (8). Low glycemic criteria also showed an increase in birth interventions and pharmacological treatment which may have subsequent negative effects (8). Furthermore, a gestational diabetes diagnosis has negative emotional and financial consequences, especially in mothers with culturally different dietary practices (9). Anxiety over a gestational diabetes diagnosis is common and some pregnant women resort to developing restrictive eating disorders to help them cope with the diagnosis (9).
Management options
Following a gestational diabetes diagnosis, management options include optimisation of diet and lifestyle, self monitoring of blood glucose, patient education and medication if necessary (2). About 50-70% of women with gestational diabetes can manage their condition through diet and exercise alone. Nutritional recommendations include lowering high glycemic foods while ensuring regular distribution of low glycemic carbohydrates as well as meeting the other macro and micronutrient requirements of pregnancy (2). Vitamin D, probiotics, omega 3 and magnesium are shown to play a role in the prevention and management for gestational diabetes (14). Moderate exercise can assist with keeping blood glucose within range (2). In Australia, diabetes educators can help assist pregnant mothers in making appropriate dietary and lifestyle changes as well as teaching women how to self monitor diabetes (2).
Self monitoring of diabetes is necessary to help pregnant women monitor their glucose response and adjust their choices accordingly. Self monitoring usually involves a finger prick to collect a blood sample. The blood is placed on a testing strip which is entered into a glucose monitor to measure the amount of glucose in the blood. Many blood glucose monitoring devices are subsidised or provided free of charge (2). Women are advised to test their blood four times a day, before breakfast and either one or two hours after each meal (2). Treatment targets are fasting less than 5.0 mmol/L, 1 hour post meal less than 7.4 mmol/l and 2 hour post meal less than 6.7 mmol/L. Continuous glucose monitors are a recent technological development that may play a larger role for gestational diabetes self monitoring in the future. While more evidence is needed, recent data suggests that continuous glucose monitoring may be an option for the diagnosis of gestational diabetes (15) and may help to identify women with gestational diabetes at higher risk of adverse pregnancy outcomes (11).
In cases where blood glucose can not be controlled through diet and lifestyle, medications are sometimes warranted. Insulin therapy is safe and effective for both mother and foetus. Insulin is generally used as a modified multi dose regimen which enables individualised treatment for pregnant women (2). Metformin is considered safe in short term studies and can help around 50% of pregnant women achieve blood sugar control however it is not commonly used in Australia due to lack of long term safety studies (2). Other blood sugar medications are considered unsafe in pregnancy (2).
Fetal monitoring is recommended for all gestational diabetes pregnancies. Growth scans can help potentially identify macrosomia or foetal growth restriction (2). Monitoring and a discussion around timing and birth options is necessary if any complications arise (2). Infants of gestational diabetes pregnancies are more susceptible to hypoglycemia after birth and should have their blood glucose tested 1-2 hour post birth (2). In the majority of cases gestational diabetes resolves after birth, however another oral glucose tolerance test is advised at 6 weeks – 3 months post birth to assess diabetes status (2).
References:
[1]Quintanilla BS, Mahdy H. Gestational diabetes. National Library of Medicine 2019. https://www.ncbi.nlm.nih.gov/books/NBK545196/.
[2]Nankervis A, Price S, Conn J. Gestational diabetes mellitus: A pragmatic approach to diagnosis and management. Australian Journal of General Practice 2018;47:445–9. https://doi.org/10.31128/ajgp-01-18-4479.
[3]Davidson KW, Barry MJ, Mangione CM, Cabana M, Caughey AB, Davis EM, et al. Screening for Gestational Diabetes. JAMA 2021;326:531. https://doi.org/10.1001/jama.2021.11922.
[4]Information NC for B, Pike USNL of M 8600 R, MD B, Usa 20894. How is gestational diabetes diagnosed? Institute for Quality and Efficiency in Health Care (IQWiG); 2020.
[5]Metzger BE, Lowe LP, Dyer AR, Trimble ER, Chaovarindr U, Coustan DR, et al. Hyperglycemia and adverse pregnancy outcomes. The New England Journal of Medicine 2008;358:1991–2002. https://doi.org/10.1056/NEJMoa0707943.
[6]Incidence of gestational diabetes in Australia, Changing trends – Australian Institute of Health and Welfare. Australian Institute of Health and Welfare 2019. https://www.aihw.gov.au/reports/diabetes/incidence-of-gestational-diabetes-in-australia/contents/changing-trends.
[7]Hillier TA, Pedula KL, Ogasawara KK, Vesco KK, Oshiro CES, Lubarsky SL, et al. A Pragmatic, Randomized Clinical Trial of Gestational Diabetes Screening. New England Journal of Medicine 2021;384:895–904. https://doi.org/10.1056/nejmoa2026028.
[8]Crowther CA, Samuel D, McCowan LME, Edlin R, Tran T, McKinlay CJ. Lower versus Higher Glycemic Criteria for Diagnosis of Gestational Diabetes. New England Journal of Medicine 2022;387:587–98. https://doi.org/10.1056/nejmoa2204091.
[9]Craig L, Sims R, Glasziou P, Thomas R. Women’s experiences of a diagnosis of gestational diabetes mellitus: a systematic review. BMC Pregnancy and Childbirth 2020;20. https://doi.org/10.1186/s12884-020-2745-1.
[10]Huhn EA, Rossi SW, Hoesli I, Göbl CS. Controversies in Screening and Diagnostic Criteria for Gestational Diabetes in Early and Late Pregnancy. Frontiers in Endocrinology 2018;9. https://doi.org/10.3389/fendo.2018.00696.
[11]Liang X, Fu Y, Lu S, Menglei Shuai, Miao Z, Gou W, et al. Continuous glucose monitoring-derived glycemic metrics and adverse pregnancy outcomes among women with gestational diabetes: a prospective cohort study. The Lancet Regional Health – Western Pacific 2023;39:100823–3. https://doi.org/10.1016/j.lanwpc.2023.100823.
[12]Asadi M, Zahedi F, Mahbube Ebrahimpur, Bagher Larijani. Ethical challenges in gestational diabetes. Journal of Medical Ethics and History of Medicine 2023. https://doi.org/10.18502/jmehm.v16i6.13470.
[13]Beilby H, Yang F, Gannon B, McIntyre HD. Cost-effectiveness of gestational diabetes screening including prevention of type 2 diabetes: application of the GeDiForCE model in Australia. The Journal of Maternal-Fetal & Neonatal Medicine 2021:1–8. https://doi.org/10.1080/14767058.2021.1973415.
[14] Ibrahim I, Bashir M, Singh P, Al Khodor S, Abdullahi H. The Impact of Nutritional Supplementation During Pregnancy on the Incidence of Gestational Diabetes and Glycaemia Control. Frontiers in Nutrition 2022;9. https://doi.org/10.3389/fnut.2022.867099.
[15] Kusinski LC, Brown J, Hughes DJ, Meek CL. Feasibility and acceptability of continuous glucose monitoring in pregnancy for the diagnosis of gestational diabetes: A single-centre prospective mixed methods study. PloS One 2023;18:e0292094. https://doi.org/10.1371/journal.pone.0292094.
