Abstract: We included three trials, all in women with type 1 diabetes (223 women and babies). All three trials were at high risk of bias due to lack of blinding, unclear methods of randomisation and selective reporting of outcomes. Two trials compared very tight (3.33 to 5.0 mmol/L fasting blood glucose (FBG)) with tight-moderate (4.45 to 6.38 mmol/L) glycaemic control targets, with one trial of 22 babies reporting no perinatal deaths orserious perinatal morbidity (evidence graded low for both outcomes). In the same trial, there were two congenital anomalies in the very tight, and none in the tight-moderate group, with no significant differences in caesarean section between groups (risk ratio (RR) 0.92, 95% confidence interval (CI) 0.49 to 1.73; evidence graded very low). In these two trials, glycaemic control was not significantly different between the very tight and tight-moderate groups by the third trimester, although one trial of 22 women found significantly less maternal hypoglycaemia in the tight-moderate group. In a trial of 60 women and babies comparing tight (≤ 5.6 mmol/L FBG); moderate (5.6 to 6.7 mmol/L); and loose (6.7 to 8.9 mmol/L) glycaemic control targets, there were two neonatal deaths in the loose and none in the tight or moderate groups (evidence graded very low). There were significantly fewer women with pre-eclampsia (evidence graded low), fewer caesarean sections (evidence graded low) and fewer babies with birthweights greater than 90th centile (evidence graded low) in the combined tight-moderate compared with the loose group. The quality of the evidence was graded low or very low for important outcomes, because of design limitations to the studies, the small numbers of women included, and wide confidence intervals crossing the line of no effect. Many of the important outcomes were not reported in these studies. In a very limited body of evidence, few differences in outcomes were seen between very tight and tight-moderate glycaemic control targets in pregnant women with pre-existing type 1 diabetes, including actual glycaemic control achieved. There is evidence of harm (increased pre-eclampsia, caesareans and birthweights greater than 90th centile) for 'loose' control (FBG above 7 mmol/L). Future trials comparing interventions, rather than glycaemic control targets, may be more feasible. Trials in pregnant women with pre-existing type 2 diabetes are required.

Plain-language summary: We found three small trials (in total 223 pregnant women with type 1 diabetes) looking at different blood glucose targets: very tight, tight, moderate, and loose. The quality of the studies and therefore the strength of the evidence was very low or low, so future research may change the results. There were very few differences between very tight and tight-moderate blood glucose targets in two trials, although there were more cases of low blood glucose (hypoglycaemia) and longer hospital stays for women who had very tight blood glucose control. A single trial compared tight, moderate, and loose blood glucose targets. In the loose target group, more women had pre-eclampsia, and there were more caesareans and large babies. There were few differences between the tight and moderate groups, although more women in the tight control group had low blood glucose in the first half of pregnancy. The evidence does not show much difference between moderate, tight and very tight blood glucose targets, although a loose blood glucose target may be worse for mothers and babies. However, the studies were small and the evidence is weak, so we do not yet know the best blood glucose target for women who have diabetes before becoming pregnant.