Three commonly-used pharmacological options are available to treat gestational diabetes (GDM) that cannot be managed with lifestyle interventions alone: insulin, glyburide, and metformin. However, there is little international consensus on optimal management strategies. We aimed to compare the impacts of pharmacological treatments for GDM on birthweight, independent of maternal glycaemic control. PubMed, OVID EMBASE, Medline, and Cochrane were systematically searched (inception-July 2019). Outcomes of GDM-affected pregnancies randomised to treatment with any of metformin, glyburide, or insulin were included. Studies including women with pre-existing or no diabetes were excluded, as were trials with no pharmacological comparison group. 33 studies (n= 4944 pregnancies) met eligibility criteria. Birthweight was highest following treatment with glyburide (+58.2g ±47.1g, I2=43%, p< 0.05) and lowest with metformin (-73.9g ±40.2g, I2=38%, p< 0.01) compared to insulin. Macrosomia rates were 38% higher in glyburide-exposed babies (OR 1.38, 95%CI:1.01-1.89; I2=31%, p< 0.05), and 40% lower in metformin-exposed babies (OR 0.60, 95%CI:0.45-0.79, I2=5%, p< 0.01) compared to insulin-exposed babies. There were no significant differences in maternal hyperglycaemia indices between treatment groups (random and fasting blood glucose, HbA1c). Maternal weight gain was lowest in the metformin-treated group, by 1.3kg (±0.4, p< 0.001) compared to insulin and 2.2kg (±1.8kg, p< 0.01) compared to glyburide. Keeping fetal growth within normal parameters is a key aim of GDM management, and is often assumed to be directly indexed to maternal glucose levels. We demonstrate that, despite equivalent maternal glucose control, glyburide-exposed babies are larger and metformin-exposed babies are smaller at birth than insulin-treated babies. Particularly in light of increasing international endorsement of oral therapies in treatment of GDM, there is an urgent need to understand the mechanisms other than maternal hyperglycaemia by which oral antihyperglyaemic agents influence birthweight and fetal growth.