The relationship between interpregnancy interval and adverse birth outcomes (i.e., low birth weight, preterm birth, and small size for gestational age) was examined in three recent studies conducted in Utah and Michigan of the United States. These studies were conducted among different populations, used different study designs (i.e., cross-sectional and retrospective cohort designs), and addressed several other methodological limitations in the previously published literature. In addition, the data were stratified by, and controlled for, several maternal reproductive risk factors. A J-shaped relationship between interpregnancy interval and adverse birth outcomes was observed in all three studies. The risk for adverse birth outcomes is lowest when the interpregnancy interval was 18-23 months and increased when the interval departed from 18-23 months. This J-shaped relationship existed at levels of maternal reproductive risk factors and after these risk factors were controlled for using logistic regression. Based on the consistency of the findings from all three studies, it appears that the J-shaped relationship between interpregnancy interval and adverse birth outcomes is causal. This information can be used by health care providers and public health programs to counsel and educate women who recently gave births on reducing the risk for adverse birth outcomes by means of appropriate pregnancy spacing.