Miscarriage is the most common complication in early pregnancy. It was recently reported in mice that miscarriage can be prevented through the administration of niacin. We conducted a prospective, exploratory pilot study involving 24 women who were less than 14 weeks pregnant. Neither niacin intake (P = 0.24) nor urinary vitamin B3 measured as the 1-methyl-5-carboxylamide-2-pyridone/N-1-methylnicotinamide (2-pyr/MNA) ratio (P = 1.00) predicted miscarriage. However, the difference in mean 2-pyr/MNA ratios between women who miscarried and controls suggests there may be a threshold niacin level protective in miscarriage prevention warranting further investigation.