Background: The optimal upper and lower limits of blood pressure in preterm infants are not known. Exceeding these thresholds may contribute to intraventricular hemorrhage (IVH). Methods: Preterm infants born ≤30 weeks GA were identified. Infants had continuous measurement of mean arterial blood pressure (MABP) for 7 days and cranial ultrasound imaging. IVH was classified as severe IVH (grade 3/4), no severe IVH (no IVH; grade 1/2), or no IVH. Mean ± SEM MABP values from hours 1–168 were calculated and sorted into bins 2 mm Hg wide. The normalized proportion of each recording spent in each bin was then calculated. Candidate limits were identified by comparison of MABP distribution in those with severe IVH vs. those without severe IVH. Results: Eighty-five million measurements were made from 157 infants. Mean EGA was 25.2 weeks; mean BW was 749 g; 65/157 female; inotrope use in 59/157; grade 3/4 IVH in 29/157. Infants with severe IVH spent significantly more time with extreme MABP measurements (<23 mm Hg or >46 mm Hg) compared to those without severe IVH (12% vs. 8% of recording, p = 0.02). Conclusions: Infants who developed severe IVH had substantially more unstable MABP and spent a significantly greater period of time with MABP outside of the optimal range.