What is the most effective blood sugar range to guide treatment for women who develop gestational diabetes mellitus in their pregnancy?

What is the issue?

Up to a quarter of pregnant women develop gestational diabetes mellitus (GDM), depending on their ethnicity and the diagnostic criteria used. GDM is defined as high blood sugar levels (hyperglycaemia) during pregnancy and is associated with an increased risk of developing high blood pressure (hypertension) and protein in the urine during pregnancy (pre-eclampsia). These women are more likely to have a caesarean birth and postnatal depression, and to develop type 2 diabetes and cardiovascular disease later in life. The high blood sugar levels that are associated with GDM usually return to normal after the birth, but women with GDM are at risk of developing GDM in future pregnancies. Babies whose mothers have been diagnosed with GDM are at an increased risk of having a birthweight greater than 4000 g, increased risk of birth trauma because of their size, and development of breathing difficulties after birth. These babies are also at higher risk of future obesity and type 2 diabetes.

Why is this important?

Women with GDM are treated with the aim of controlling high maternal blood sugar levels and reducing the risks of GDM for the mother and the baby. Blood sugar control is monitored by measuring blood sugar concentrations to ensure they are maintained within a predefined level or range. The blood sugar results are usually obtained by the mother using a finger prick to collect a drop of her blood on a test strip, which is inserted into a small machine (a glucometer) that reads the sugar level of the blood on the test strip. The glucometer reading alerts the pregnant woman to her current blood sugar level and is used to guide her treatment, for example how many units of insulin she requires before eating. However, the most effective blood sugar range to aim for and guide treatment in pregnant women with newly diagnosed GDM is currently unclear.

What evidence did we find?

This is an update on a review completed in 2016. We searched for evidence on 26 September 2022 for randomised controlled trials (a type of study where participants are randomly assigned to one of two or more treatment groups) that compared different blood sugar ranges in women with GDM and assessed the impact on mother and infant health. We found nine reports from three different trials in this updated review, amounting to a total of four included studies. Each trial compared two blood sugar ranges, one tighter (lower blood sugar targets), and the other less-tight (higher blood sugar targets), and reported on health outcomes for the pregnant woman and her baby.

We found that there may be an increase in the risk of the mother developing high blood pressure and protein in the urine during pregnancy with a lower blood sugar target. We found that there is unlikely to be a difference between blood sugar ranges in rates of caesarean birth or induction of labour. The trials did not report any data for the following outcomes for mothers: subsequent development of type 2 diabetes for the mother, trauma to the perineum, return to pre-pregnancy weight, and postnatal depression.

We are uncertain whether there is any difference in the risk of death for the baby as there were very few deaths in the studies. The evidence shows there is likely no change in the blood sugar levels of the baby and may be no change in the body fat percentage of the baby. The trials did not provide any data for the other main outcomes: long-term risk of diabetes in the baby and risk of disability in the baby.

Lower blood sugar targets likely result in an increase in the use of drug therapy (insulin, metformin, or glyburide) and may result in a large decrease in adherence to treatment.

Limitations of the evidence

There is some uncertainty in the findings due to a lack of information about how some studies were designed and reported and because for some outcomes there was information from only one study.

What does this mean?

This review found that there is not yet enough evidence from randomised trials to determine the best blood sugar range for improving health for pregnant women with GDM and their babies. The evidence currently points towards no increased benefit when using lower blood sugar targets. Two trials are ongoing. More high-quality studies are needed that compare different targets for blood sugar levels and assess both short- and long-term health outcomes for women and their babies to guide treatment. Studies should include women's experiences and assess health services costs.

Authors' conclusions: 

This review is based on four trials (1731 women) with an overall unclear risk of bias. The trials provided data on most primary outcomes and suggest that tighter glycaemic control may increase the risk of hypertensive disorders of pregnancy. The risk of birth of a large-for-gestational-age infant and perinatal mortality may be similar between groups, and tighter glycaemic targets may result in a possible reduction in composite of death or severe infant morbidity. However, the CIs for these outcomes are wide, suggesting both benefit and harm.

There remains limited evidence regarding the benefit of different glycaemic targets for women with GDM to minimise adverse effects on maternal and infant health. Glycaemic target recommendations from international professional organisations vary widely and are currently reliant on consensus given the lack of high-certainty evidence.

Further high-quality trials are needed, and these should assess both short- and long-term health outcomes for women and their babies; include women's experiences; and assess health services costs in order to confirm the current findings. Two trials are ongoing.

Read the full abstract...

Gestational diabetes mellitus (GDM) has major short- and long-term implications for both the mother and her baby. GDM is defined as a carbohydrate intolerance resulting in hyperglycaemia or any degree of glucose intolerance with onset or first recognition during pregnancy from 24 weeks' gestation onwards and which resolves following the birth of the baby. Rates for GDM can be as high as 25% depending on the population and diagnostic criteria used, and overall rates are increasing globally. There is wide variation internationally in glycaemic treatment target recommendations for women with GDM that are based on consensus rather than high-quality trials.


To assess the effect of different intensities of glycaemic control in pregnant women with GDM on maternal and infant health outcomes.

Search strategy: 

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register, ClinicalTrials.gov, the World Health Organization International Clinical Trials Registry Platform (26 September 2022), and reference lists of the retrieved studies.

Selection criteria: 

We included randomised controlled trials (RCTs), cluster-RCTs, and quasi-RCTs. Trials were eligible for inclusion if women were diagnosed with GDM during pregnancy and the trial compared tighter and less-tight glycaemic targets during management. We defined tighter glycaemic targets as lower numerical glycaemic concentrations, and less-tight glycaemic targets as higher numerical glycaemic concentrations.

Data collection and analysis: 

We used standard Cochrane methods for carrying out data collection, assessing risk of bias, and analysing results. Two review authors independently assessed trial eligibility for inclusion, evaluated risk of bias, and extracted data for the four included studies. We assessed the certainty of evidence for selected outcomes using the GRADE approach. Primary maternal outcomes included hypertensive disorders of pregnancy and subsequent development of type 2 diabetes. Primary infant outcomes included perinatal mortality, large-for-gestational-age, composite of mortality or serious morbidity, and neurosensory disability.

Main results: 

This was an update of a previous review completed in 2016. We included four RCTs (reporting on 1731 women) that compared a tighter glycaemic control with less-tight glycaemic control in women diagnosed with GDM. Three studies were parallel RCTs, and one study was a stepped-wedged cluster-RCT. The trials took place in Canada, New Zealand, Russia, and the USA. We judged the overall risk of bias to be unclear. Two trials were only published in abstract form. Tight glycaemic targets used in the trials ranged between ≤ 5.0 and 5.1 mmol/L for fasting plasma glucose and ≤ 6.7 and 7.4 mmol/L postprandial. Less-tight targets for glycaemic control used in the included trials ranged between < 5.3 and 5.8 mmol/L for fasting plasma glucose and < 7.8 and 8.0 mmol/L postprandial.

For the maternal outcomes, compared with less-tight glycaemic control, the evidence suggests a possible increase in hypertensive disorders of pregnancy with tighter glycaemic control (risk ratio (RR) 1.16, 95% confidence interval (CI) 0.80 to 1.69, 2 trials, 1491 women; low certainty evidence); however, the 95% CI is compatible with a wide range of effects that encompass both benefit and harm. Tighter glycaemic control likely results in little to no difference in caesarean section rates (RR 0.98, 95% CI 0.82 to 1.17, 3 studies, 1662 women; moderate certainty evidence) or induction of labour rates (RR 0.96, 95% CI 0.78 to 1.18, 1 study, 1096 women; moderate certainty evidence) compared with less-tight control. No data were reported for the outcomes of subsequent development of type 2 diabetes, perineal trauma, return to pre-pregnancy weight, and postnatal depression.

For the infant outcomes, it was difficult to determine if there was a difference in perinatal mortality (RR not estimable, 2 studies, 1499 infants; low certainty evidence), and there was likely no difference in being large-for-gestational-age (RR 0.96, 95% CI 0.72 to 1.29, 3 studies, 1556 infants; moderate certainty evidence). The evidence suggests a possible reduction in the composite of mortality or serious morbidity with tighter glycaemic control (RR 0.84, 95% CI 0.55 to 1.29, 3 trials, 1559 infants; low certainty evidence); however, the 95% CI is compatible with a wide range of effects that encompass both benefit and harm. There is probably little difference between groups in infant hypoglycaemia (RR 0.92, 95% CI 0.72 to 1.18, 3 studies, 1556 infants; moderate certainty evidence). Tighter glycaemic control may not reduce adiposity in infants of women with GDM compared with less-tight control (mean difference −0.62%, 95% CI −3.23 to 1.99, 1 study, 60 infants; low certainty evidence), but the wide CI suggests significant uncertainty. We found no data for the long-term outcomes of diabetes or neurosensory disability.

Women assigned to tighter glycaemic control experienced an increase in the use of pharmacological therapy compared with women assigned to less-tight glycaemic control (RR 1.37, 95% CI 1.17 to 1.59, 4 trials, 1718 women). Tighter glycaemic control reduced adherence with treatment compared with less-tight glycaemic control (RR 0.41, 95% CI 0.32 to 0.51, 1 trial, 395 women).

Overall the certainty of evidence assessed using GRADE ranged from low to moderate, downgraded primarily due to risk of bias and imprecision.