A number of studies have shown that children born by cesarean section have lower blood pressure during the neonatal period. The aim of this study was to investigate whether mode of delivery influenced childhood blood pressure: at age 7.5 to 8 y in a cohort of 756 children born preterm, at 7 to 9 y in a pilot study of 166 children born at term in the United Kingdom, and in a cohort of 650 Tasmanian children born at term. In the preterm cohort, systolic blood pressure was significantly lower in children born by cesarean section rather than delivered vaginally (99.3+/-10.0 versus 101.4+/-9.4 mm Hg; 95% confidence interval, -0.69 to -3.46; p = 0.003), with a significant trend to having a higher pressure in those born by breech versus forceps versus spontaneous vaginal delivery versus cesarean section. These findings were not replicated in the term cohorts. This raises the hypothesis that there is a sensitive period for programming later blood pressure by factors associated with mode of delivery and that this period does not extend to full-term.