The records of the Medical Birth Registry of Norway from 1970 through 1988 contain information on maternal health, course of delivery and health of 1,059,479 newborns. The overall prevalence at birth of neonatal hip instability (NHI) was 0.9 percent: 0.6 percent in boys and 1.4 percent in girls. In breech presentation, the rate was 4.4 percent. In vaginally delivered children, the rate was only marginally higher compared to those delivered by cesarean section. In children with a birthweight less than 2,500 g, the rate was 0.3 percent. In vertex presentation, the duration of pregnancy had no influence in boys whilst, in breech presentation, the prevalence increased up to the 39th week of gestation. In girls, the NHI rate increased with the duration of gestation, particularly in breech presentation. In first-born children, these patterns were even more obvious. The data are consistent with a hypothesis that intra-uterine mechanical factors, in combination with hormonal factors, are of importance rather than the actual trauma of vaginal delivery.