The full, clinically endorsed recommendation should be obtained from Lareb [link].   

Clinical Overview  

Metformin is used in pregnancy for the treatment of gestational diabetes mellitus and pre-gestational) type 2 diabetes. Uncontrolled diabetes increases the risk of adverse outcomes for both mother and foetus, making adequate management essential. Because pregnancy increases renal clearance of drugs and foetal exposure occurs, dose-adjustments may be necessary to maintain effective concentrations.  

Pharmacokinetics of metformin in pregnancy 

Pharmacokinetic and modelling studies show that metformin concentrations are decreased by approximately 20–35% compared to non-pregnant individuals. Meanly due to increased renal clearance. Concentrations are lowest in mid–late pregnancy and rise again postpartum. Metformin crosses the placenta and foetal concentrations can approach maternal levels. 

Efficacy and safety of metformin in pregnancy 

Metformin remains clinically effective for glycaemic control in many pregnant patients, though evidence directly linking higher dose to greater efficacy in pregnancy is lacking. To date, there is no evidence for an increased risk of congenital abnormalities or other adverse pregnancy outcomes. However, information regarding dose-related foetal safety is lacking. 

Conclusion  

Pregnancy lowers metformin exposure via increased renal elimination. Foetal levels can mirror maternal concentrations, but overall safety data is reassuring.