The prevalence of gestational diabetes mellitus (GDM) is increasing worldwide. This disease has many detrimental consequences for the woman, the unborn foetus and child. The management of GDM aims to mediate the effects of hyperglycaemia by controlling blood glucose levels. Along with pharmacology and dietary interventions, exercise has a powerful potential to assist with blood glucose control. Due to the uncertainty of risks and benefits of exercise during pregnancy, women tend to avoid exercise. However, under adequate supervision exercise is both safe and beneficial in the treatment of GDM. Therefore it is vital that exercise is incorporated into the continuum of care for women with GDM. Medical doctors should be able to refer to competently informed exercise professionals to aid in GDM treatment. It is important that exercise treatment is informed by research. Hence, the development of evidence-based guidelines is important to inform practice. Currently there are no guidelines for exercise in GDM. This review aims to assess the efficacy of exercise for the management of GDM in order to establish an exercise prescription guideline specific to the condition. It is recommended that women with GDM should do both aerobic and resistance exercise at a moderate intensity, a minimum of three times a week for 30-60 min each time.

The use of exercise as part of the continuum of treatment in patients with diabetic related disorders is accepted and widely encouraged[1]. One increasingly prevalent metabolic disorder is gestational diabetes mellitus (GDM). Although exercise prescription as treatment in this population group may be encouraged, a general exercise prescription guideline is lacking. Therefore the purpose of this review is to assess the efficacy of exercises currently being prescribed for the management of GDM in order to establish an exercise prescription guideline specific to GDM.

GDM is defined as a “carbohydrate intolerance of varying degrees of severity with onset or first recognition during pregnancy”[2]. Other variants of this definition further refer to the period of onset of hyperglycaemia, specifically within 24-28 wk of gestation[3] and a natural dispelling of the hyperglycaemic condition after child birth[4].

The earliest record of hyperglycaemia during pregnancy is from the 1960’s, when a research group led by O’Sullivan (and endocrinologist/gynaecologist) noted that hyperglycaemia in pregnant women was associated with poorer pregnancy outcomes and a higher occurrence of type 2 diabetes in the years ensuing pregnancy[4]. In 2013, the World Health Organisation (WHO) released a classification and diagnostic criteria for GDM (Table 1)[5]. The new criteria was developed to take into account a quantifiable relationship between hyperglycaemia and adverse short-term pregnancy outcomes for both mother and newborn, in light of findings from the study of Hyperglycaemia and Adverse Pregnancy Outcomes (HAPO)[6]. This diagnostic guideline can be used throughout pregnancy and can distinguish GDM from diabetes mellitus in pregnancy (Table 1)[5]. However, as the new criterion recommends lower fasting plasma glucose levels than what has been previously used, the WHO anticipated as increase in the number of pregnant women diagnosed with GDM[5]. The effects of these criteria changes are yet to be evaluated, especially in the area of pregnancy outcomes.

There is a range of risk factors that increase the chance of developing GDM. Ethnicity may play a role in GDM development as elevated incidences have been reported in certain ethnic subgroups. In the United States epidemiological studies have reported a higher incidence of GDM in African Americans, Native Americans, Hispanics and Orientals than in non-white Hispanic women[7-9]. In Australia, the prevalence of GDM was found to be higher in Aboriginal Australians as well as women who were born in Asia and India[10-12].

In Europe, a large scale epidemiological study (n = 11205) in London found that women of ethnic minority groups had higher prevalence of GDM[13]. When compared to White women, relative risks ranged from 3.1 for Black, 5.9 for miscellaneous, 7.6 for South Asian and 11.3 for Indian women[13]. Asian women living in Asia have less proportional incidence than Asian born women living in other continents[14]. Indian women living in urban areas have greater observed incidence that those living in rural areas of India[15].

Furthermore, non-modifiable risk factors include greater maternal age (defined as 35 year of age plus)[16,17], polycystic ovarian syndrome, family history of diabetes mellitus and pregnancy induced hypertension[17]. Incidence of GDM has also been 30%-60% greater in women who have experienced the disease in a previous pregnancy[18-22]. In a 16 year longitudinal retrospective study of 651 Canadian women who had GDM during their first pregnancy, 35% developed GDM in their second pregnancy[23]. Greater pre-maternal weight was a strong predicting factor of GDM re-occurrence in ensuing pregnancies[23].

This finding leads into a discussion of lifestyle related factors that are largely modifiable. Factors such as overweight and obesity can be modified. Weight gain during pregnancy was investigated using a nested-case control study of 1145 women[24]. Findings suggested that subjects with greater weight gain during pregnancy (0.27-0.41 kg/wk or more) had an increased risk of developing GDM by 43%-74%. This effect was further exacerbated in overweight and obese women[24]. Furthermore poor diet and nutrition are reported as mediators of increasing maternal weight and risk of GDM[25-27]. Diets particularly high in refined sugars, with a high glycaemic index and fat content have been thought to increase the risk of GDM and hypertension during pregnancy[25,28]. However, the majority of studies reporting these findings are small and have not been able to provide conclusive evidence on the role of diet in increasing the risk of GDM[25,28]. Even so, one large prospective study (n = 13475), investigated the consumption of sugar sweetened beverages and incidence of GDM. After a 10 year follow up and 860 cases of GDM, excessive consumption (≥ 5 servings a week) of sugar sweetened cola was found to increase the risk of GDM by 22%[29].

In a population based longitudinal study of 824 women, hypertension was found to increase relative risk of developing GDM up to twice as much (relative risk 2.03) as women without hypertension[30]. The relationship between hypertension and GDM was confirmed in a Danish study of 215 women, in which higher rates of GDM was found in those with hypertension[31]. Literature surrounding the pathophysiology of hypertension and gestational diabetes does not clearly underpin the mechanisms that cause such complications. However, it has been suggested that hypertension during pregnancy may heighten insulin resistance, alter immune responses and inflammatory pathways[32]. These responses further encourage hypertension and hyperglycaemia, reminiscent of pathways in the metabolic syndrome[32,33].

Sedentary behaviour and lifestyles characterised by low levels of physical activity have also been shown to increase the risk of GDM. In the largest reported study documenting physical activity and sedentary behaviour in relation to risk of GDM, Zhang et al[34] reviewed cases of 21765 women’s pregnancies, 1428 of whom had GDM. They controlled for dietary factors and other covariates and found inverse relationships between vigorous activity, higher weekly physical activity levels and risk of GDM[34]. Vigorous activity and brisk walking both had protective effects against developing GDM (RR = 0.77 and RR = 0.66 for each respectively). Women who were sedentary had a greater that two fold increase in developing GDM (RR = 2.30)[34]. A more recent randomised control trial involving pregnant women had an exercise group (n = 40) which engaged in moderate land based resistance activity combined with aquatic aerobic activity[35]. The control group (n = 43) was not given physical activity advice or exercise supervision. During 24-28 wk of gestation the women underwent a 50g maternal glucose screening test of which the exercise group had significantly better glucose levels (5.76 ± 1.13) that the control group (7.05 ± 1.73 mmol/L)[35]. A case-control study by Dempsey et al[36]found a large reduction in GDM diagnosis in non-diabetic women who participated in recreational physical activity in the first 20 wk of pregnancy by 48%-78%. Further prospective studies of 909 non-diabetic American women, found that recreational exercise leading up to pregnancy, when performed for at least one year can induce a 56%-76% reduction in the risk of developing GDM[37]. A large meta-analyses investigating seven pre-pregnancy and five early pregnancy studies found that pre-pregnancy exercise gave a pooled odds ratio of 0.45, a high protective effect against GDM[38]. Furthermore, exercise in early pregnancy was also significantly protective of developing GDM with an odds ratio of 0.76[38]. For a summary of risk factors see Figure 1.

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Figure 1 Risk factors for gestational diabetes mellitus. GDM: Gestational diabetes mellitus.

GDM affects the health of the women, the foetus and even after birth, the baby or child. Hyperglycaemic placental environments increase the risk of traumatic pregnancies influenced by macrosomia (larger than usual birth weight)[39,40]. This in turn increases the risk of the baby having shoulder damage during birth[40]. Macrosomia can be further exacerbated by excess levels of insulin circulating in the placenta[3,41]. This is due to the increased growth effects of insulin on the foetus[41]. Therefore care must be taken when prescribing anti-glycaemic maternal medication. The uses of different types of medications are still being investigated and there is a body of evidence yet to be filled regarding the direct impact of anti-glycaemic agents on foetal health. However, reviews have suggested that the use of insulin secretagoues is the safest form of pharmacological treatment in GDM, as they have little to no perfusion across the placenta[41-43]. However, oral hypoglycaemic agents including metformin and glyburide have been used as alternative pharmacological treatment to insulin therapy[44,45].

Further complications for the baby directly after birth include acute breathing difficulty, jaundice and nerve palsy[46-48]. Babies born of women with poorly controlled GDM are likely to experience hypoglycaemia after birth due to an immediate impairment of environmental glucose[46,49]. Because of the complications presented, babies are more likely to be admitted to intensive care units and endure longer hospital stay times[48,49]. As the child grows they are more likely to develop obesity as well as metabolic disorders such as type 2 diabetes[50,51].

Traumatic births caused by the compounding effects of GDM also present complications for the women’s immediate health[47]. Women are more likely to undergo an emergency caesarean section, particularly with the presence of preeclampsia[49]. Preeclampsia is a combination of high blood pressure and excessive protein in urine, also the second leading cause of maternal death[49]. Due to the increased size of the baby, they are also more likely to have a longer active labour times, instrumental vaginal delivery as well as third degree perineal trauma and tearing[49,51]. These traumatic events generally lead to post-partum haemorrhage and hence longer hospital stay times[49]. Furthermore, women with GDM have greater weight retention post-partum leading to a greater risk of overweight and obesity[51,52]. This has also been shown to correlate with increased risk of developing chronic hypertension, type 2 diabetes and GDM is ensuing pregnancies[49,51,52].

Because of the vast effects of GDM on the health of both mother and child, the management of GDM is critical in minimising the effects of hyperglycaemia during pregnancy.

The primary aim of GDM management is to optimise glucose control and improve pregnancy outcomes[53]. Generally speaking the initial management of GDM involves diet modification and implementation of an exercise regime[49,54]. If adequate glucose control has not been achieved, the woman will generally be prescribed anti-diabetic medications to directly reduce blood glucose levels maternally and hence indirectly for the foetus[42].

Although it is not a treatment method, constant evaluation of the foetus’ health and development is recommended to continually assess for deformities and macrosomia[54]. This includes foetal surveillance using ultrasound and Doppler of umbilical blood flow measurement[49,54,55]. In a large randomly controlled trial, 1000 women diagnosed with GDM were randomly assigned gestational diabetes care (n = 490) or routine care (n = 510). The gestational diabetes care group received dietary advice, blood glucose monitoring and insulin therapy. The control group received no standard pregnancy care. Pregnancy outcomes were assessed and after controlling for various factors including ethnicity and age, the diabetes care group had lower rates of serious perinatal complications (1% vs 4%)[49]. However, this intervention did not include maternal exercise or education. There were also no protective effects on admittance to the intensive care unit, induced labour or risk of caesarean delivery. A more recent and larger meta-analysis by Poolsup et al[56] investigated ten studies regarding the outcomes of GDM treatment methods including pharmaceutical and dietary care. It was found that such treatment significantly reduced the risk of macrosomia (RR = 0.47), shoulder damage during birth (RR = 0.42) and gestational hypertension (RR = 0.68). However alongside the absence of exercise from these treatments, there was no change in the risk of neonatal mortality, neonatal hypoglycaemia, birth trauma, premature births, preeclampsia, caesarean section and induced labour[56].

The recommended quintet approach for GDM (Figure 2) includes maternal education, diet modification, exercise, pharmacology and foetal surveillance. This has been developed in light of the confounding evidence of increased positive pregnancy outcomes in studies of these areas independently. A study investigating the efficiency and practicality of this complete model of maternal and foetal care is yet to be investigated.

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Figure 2 Gestational diabetes mellitus quintet management model.

In regards to evidence specific to GDM treatment, exercise has been shown to be an effective tool in glucose control which may prevent, reduce or delay the need for insulin[57,58]. Tight glucose control is considered especially important in the gestational patient, considering the increased risk of poor health outcomes for both mother and child in the presence of hyperglycaemia. For future ramifications, it has been found that any degree of abnormal glucose homeostasis during gestation can also independently predict diabetic re-occurrence in women with GDM[57].

Although currently there are no GDM specific exercise prescription guidelines published, research has been conducted in general pregnancy and exercise. There has been no suggestion of the need to any extra precautions than the precautions taken when exercising pregnant women without GDM. However, considering the added hyperglycaemia, the same considerations and precautions concerning type 2 diabetes should also be considered when exercising women with GDM. Hence the FITT (frequency, intensity, time/duration and type) principles of exercise examined and presented in this review will take into consideration research including women with GDM, without GDM and type 2 diabetes. Given the lack of large cohort studies implementing exercise as management of GDM, recommendations have been drawn from exercise in pregnancy guidelines and exercise in type 2 diabetes guidelines.

For women who were previously sedentary before pregnancy, it is advised that they consult their medical practitioner who may asses their suitability to exercise. It is recommended that a suitability qualified exercise physiologist be actively involved in exercise prescription and delivery. This person would then be able to liaise with the medical practitioner to apply advice regarding suitability. As with any clinical population there are some contraindications to exercise in pregnancy. The absolute contraindications as recommended by the American Congress of Obstetricians and Gynaecologists (ACOG) are medical conditions which may be exacerbated by engaging in exercise. It is important that to educate patients that these conditions are not caused by exercise and until the condition is stabilised, they should not engage in exercise. Furthermore, ACOG has developed relative contraindications for engaging in aerobic exercise during pregnancy. Clinical knowledge and expertise must be applied when assessing each individual situation in regards to exercising with relative contraindications. It is recommended that advice from the medical practitioner is carefully interpreted by the exercise physiologist to determine if the benefits out way any risks of exercise. Both absolute and relative contraindications for exercise in pregnancy are summarised in Table 2.

Although there are vast benefits from exercising during pregnancy, some precautions need to be observed to encourage safety for both mother and child. As previously noted, the ACOG has advised against some forms of exercise including the following: recreational sports with increased risk of forceful contact or falling (i.e., basketball, rugby, horseback riding and gymnastics), exercising in a supine position after the first trimester, motionless standing and scuba diving[69,75].

Furthermore, if any of the following warning signs occur, it is advised that exercise should be terminated: vaginal bleeding, dizziness, headache, chest pain, muscle weakness, preterm labour, decreased foetal movement, amniotic fluid leakage, calf pain or swelling and dyspnoea without exertion. It is important to regain stability of the mother’s and foetus’ condition as soon as possible. Treatment and advice from a medical practitioner should be incorporated in the exercise program[69,75].

Exercising individuals with impaired fasting glucose presents its own sense of challenges and special considerations. Preventative measures should be in place to minimise the risk of an adverse event occurring and not prevent individuals from exercising. One especially important possible complication, however rare is hypoglycaemia. It is suggested that continual self-monitoring of blood glucose levels with physician consultation should be encouraged. Furthermore, if at pre-exercise the blood glucose level is ≤ 4.0 mmol/L this should be considered low and exercise should not begin till administration of some long and short acting glucose in food or drink[85]. In order to take advantage of the hyperglycaemic effect of food, it may be advantageous to exercise an hour after a meal[85]. It also may be important to consider taking insulin medication well before exercise to further reduce the risk of hypoglycaemia.

All pregnant women should engage in physical activity and may benefit from planned and programmed exercise. Women with GDM have extra physiological challenges that when left unattended to, have the potential to increase negative pregnancy outcomes for both mother and child. When used effectively, exercise can be used as a tool of treatment as part of the continuum of care for women with GDM. General guidelines encourage these women to engage in moderate intensity aerobic and strength training along with recreational physical activity. Exercise programs should be tailored by appropriately trained and qualified professionals (e.g., Exercise Physiologists) who have knowledge, training and experience to understand the individual’s physiological needs and associated risks.