An immune reaction initiated by paternal antigens may be necessary for healthy placental development, pregnancy maintenance and infant growth. An inadequate immune response may result in intrauterine growth retardation (IUGR). We hypothesised that a change in paternity may interfere with the immune response and cause poor placentation with resultant IUGR. In this paper we examine the risk of IUGR associated with changes in paternity. We used the Utah Successive Pregnancies Data Set that contains information on women across their pregnancies. We restricted the analysis to 141,817 women with two or three pregnancies. Women who did not have an IUGR infant in the previous pregnancy were at a 20-30% greater risk of developing IUGR if they had changed partners. Women who had a previous IUGR infant were at no increased risk for IUGR after a change in paternity. These results may point to an immune mechanism or may be as a result of residual confounding of unmeasured risk factors for IUGR. Further studies with data that contain more sociodemographic and biological risk factors for IUGR are necessary to exclude residual confounding.