During pregnancy a mother experiences increased metabolic demands to meet the needs of the fetus. A mismatch between these demands and nutrient intake can result in a host of developmental abnormalities to the fetus and health risks to the mother. Several studies have reported strong correlations between deficiency of the essential mineral, magnesium (Mg2+), and many pregnancy complications, including intrauterine growth restriction, preeclampsia, gestational diabetes, and preterm delivery. Mg2+ also impacts fetal programming and disease presentation in childhood and adulthood, showing that aberrant Mg2+ levels in utero have far reaching consequences. Unfortunately, there is no established clinical range of normal serum Mg2+ levels, which makes it challenging to identify mothers and/or fetuses at risk of adverse effects. In this review, we evaluated recently published data to identify a range of serum Mg2+ concentrations that may reflect chronic Mg2+ insufficiency (0.7- 0.85 mmol/L). We then evaluated independent studies that reported on the relationship between Mg2+ levels and pregnancy outcomes to assess whether this cutoff may help identify patients at risk for adverse events and inform therapeutic strategies. Our literature review showed that chronic Mg2+ insufficiency coupled with a molar ratio of calcium (Ca2+) to Mg2+ greater than 3 may indicate increased risk to mother or fetus. Given the high social and economic burdens of pregnancy complications, nutritional supplementation that includes Mg2+ at all stages of pregnancy may be a safe and cost-effective way to mitigate the risk of adverse outcomes for mother and child.