Metformin is the first line treatment for many people with type 2 diabetes mellitus (T2DM) and gestational diabetes mellitus (GDM) to maintain glycaemic control. Recent evidence suggests metformin can cross the placenta during pregnancy, thereby exposing the fetus to high concentra-tions of metformin and potentially restricting placental and fetal growth. Offspring exposed to metformin during gestation are at increased risk of being born small for gestational age (SGA) and show signs of ‘catch up’ growth and obesity during childhood which increases their risk of future cardiometabolic diseases. The mechanisms by which metformin impacts on fetal growth and long-term health of the offspring remain to be established. Metformin is associated with ma-ternal vitamin B12 deficiency and antifolate like activity. Vitamin B12 and folate balance is vital for one carbon metabolism, essential for DNA methylation and purine/pyrimidine synthesis of nucleic acids. Folate: vitamin B12 imbalance induced by metformin may lead to genomic instabil-ity and aberrant gene expression, thus promoting fetal programming. Mitochondrial aerobic res-piration may also be affected, thereby inhibiting placental and fetal growth, and suppressing mammalian target of rapamycin (mTOR) activity for cellular nutrient transport. Vitamin sup-plementation, before or during metformin treatment in pregnancy, could be a promising strategy to improve maternal vitamin B12 and folate levels and reduce the incidence of SGA births and childhood obesity. Heterogenous diagnostic and screening criteria for GDM and the transient nature of nutrient biomarkers have led to inconsistencies in clinical study designs to investigate the effects of metformin on folate: vitamin B12 balance and child development. As rates of diabetes in pregnancy continue to escalate, more women are likely to be prescribed metformin thus it is of paramount importance to improve our understanding of metformin’s transgenerational effects to develop prophylactic strategies for the prevention of adverse fetal outcomes.