Objective To investigate the distribution of known factors for preterm birth (PTB) by severity of maternal underweight; to investigate the risk adjusted relationship between severity of underweight and PTB and to assess if the relationship differed by gestational age. Design Retrospective cohort study. Setting State of California, USA. Methods Maternally linked hospital and birth certificate records of 950,356 California deliveries in 2007–2010 were analyzed. Singleton live births of women whose pre-pregnancy body mass index (BMI) was underweight (<18.5 kg/m2) or normal (18.50–24.99 kg/m2) were analyzed. Underweight BMI was further categorized as: severe (<16.00), moderate (16.00–16.99) or mild (17.00–18.49). PTB was grouped as 22–27, 28–31, 32–36 or <37 weeks (compared with 37–41 weeks). Adjusted multivariable Poisson regression modeling was used to estimate relative risk for PTB. Main outcome measures Risk of PTB. Results 72,686 (7.6%) women were underweight. Increasing severity of underweight was associated with increasing percent PTB: 7.8% (n=4421) in mild, 9.0% (n=1001) in moderate and 10.2% (475) in severe underweight. The adjusted relative risk of PTB also significantly increased: aRR=1.22 (95%CI: 1.19–1.26) in mild, aRR=1.41 (95%CI: 1.32–1.50) in moderate and aRR=1.61 (95%CI: 1.47–1.76) in severe underweight. These findings were similar in spontaneous PTB, medically indicated PTB, and the gestational age groupings. Conclusion Increasing severity of maternal pre-pregnancy underweight BMI was associated with increasing risk adjusted PTB at <37 weeks. This increasing risk was of similar magnitude in spontaneous and medically indicated births and in preterm delivery at 28–31 and at 32–36 weeks of gestation.