Fifty-seven prenatal patients with no known exposure to the element mercury, or any of its compounds, were observed for change in whole blood total mercury concentration from the initial prenatal clinic examination through delivery and postpartum hospitalization. On hospital admission for labor and delivery, whole blood total mercury averaged 1.15 parts per billion (ppb), compared to 0.79 ppb from the first prenatal clinic visit; these levels represent a 46% increase and significant difference in maternal concentration of a substance previously recognized for its peculiar ease at crossing the placental barrier. Previous stillbirths, as well as history of birth defects, exhibited significant positive correlation with background mercury levels. Search of the literature of the last 5 years revealed no other report of cohort heavy metal surveillance throughout pregnancy.