The incidence of preterm birth in developed countries is increasing, and in some countries, including the United States, it is almost as high as in developing countries. Demographic changes in women becoming pregnant can account for only a relatively small proportion of the increase. A significant proportion of spontaneous preterm birth continues to be of unknown cause. Experimental data from animal studies suggesting that maternal undernutrition may play a role in spontaneous, noninfectious, preterm birth are supported by observational data in human populations, which support a role for maternal prepregnancy nutritional status in determining gestation length. In addition, intakes or lack of specific nutrients during pregnancy may influence gestation length and thus the risk of preterm birth. As yet, the role of paternal nutrition in contributing to gestation length is unexplored.