Metformin serum concentrations during pregnancy and post partum – A clinical study in patients with polycystic ovary syndrome

BACKGROUND AND OBJECTIVES: Metformin is used to treat gestational diabetes. It is also used to treat women with polycystic ovary syndrome and has been shown to prevent late miscarriage and preterm birth. However, increased renal clearance during pregnancy causes a decline in serum concentrations of metformin. The aim of this study was to explore the time course of the pregnancy‐related changes in metformin pharmacokinetics and the return to the non‐pregnant state. METHOD: A subgroup of women in the PregMet2 study (n = 73) agreed to provide serum samples at three time‐points in pregnancy (gestational weeks 19, 28 and 32) and once in post partum, (either 2, 4 or 8 weeks after delivery). Serum metformin concentrations were compared using a four‐parameter logistic model. FINDINGS: The mean steady‐state serum concentration of metformin during pregnancy was 9.39 μmoL/L, whereas the post partum concentration was 12.36 μmoL/L, an increase of 32% (p = 0,019). This change took place already during the first 2 weeks post partum. CONCLUSION: Clinicians who treat pregnant women with metformin should be aware of the significant decrease in metformin concentration mediated by pregnancy, and the rapid increase after delivery, as it may impact both the therapeutic efficacy and the risk of adverse drug reactions.