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Background: In both the biomedical and public health literature, the risk for preterm birth has been linked to maternal racial/ethnic background, in particular African-American heritage. Despite this well-documented health disparity, the relationship of comorbid conditions, such as chronic hypertension, to maternal race/ethnicity and preterm birth has received relatively limited attention in the literature.Objective: The objective of the study was to evaluate the interaction between chronic hypertension and maternal racial/ethnic background on preterm birth.Study Design: This is a retrospective cohort study of singleton pregnancies among women who delivered between 2002 and 2015 at the University of California, San Francisco. The associations of chronic hypertension with both spontaneous and medically indicated preterm birth were examined by univariate and multivariate logistical regression, adjusting for confounders including for maternal age, history of preterm birth, maternal body mass index, insurance type (public vs private), smoking, substance abuse, history of pregestational diabetes mellitus, and use of assisted reproductive technologies. The interaction effect of chronic hypertension and racial/ethnicity was also evaluated. All values are reported as odds ratios, with 95% confidence intervals and significance set at P = .05.Results: In this cohort of 23,425 singleton pregnancies, 8.8% had preterm deliveries (3% were medically indicated preterm birth, whereas 5.5% were spontaneous preterm births), and 3.8% of women carried the diagnosis of chronic hypertension. Chronic hypertension was significantly associated with preterm birth in general (adjusted odds ratio, 2.74, P < .001) and medically indicated preterm birth specifically (adjusted odds ratio, 5.25, P < .001). When evaluating the effect of chronic hypertension within racial/ethnic groups, there was an increased odds of a preterm birth among hypertensive, African-American women (adjusted odds ratio, 3.91, P < .001) and hypertensive, Asian-American/Pacific Islander women (adjusted odds ratio, 3.51, P < .001) when compared with their nonhypertensive counterparts within the same racial/ethnic group. These significant effects were also noted with regard to medically indicated preterm birth for hypertensive African-American women (adjusted odds ratio, 6.85, P < .001) and Asian-American/Pacific Islander women (adjusted odds ratio, 9.87, P < .001). There was no significant association of chronic hypertension with spontaneous preterm birth (adjusted odds ratio, 0.87, P = .4).Conclusion: The effect of chronic hypertension on overall preterm birth and medically indicated preterm birth differs by racial/ethnic group. The larger effect of chronic hypertension among African-American and Asian/Pacific Islander women on medically indicated and total preterm birth rates raises the possibility of an independent variable that is not captured in the data analysis, although data regarding the indication for medically indicated preterm delivery was limited in this data set. Further investigation into both social-structural and biological predispositions to preterm birth should accompany research focusing on the effect of chronic hypertension on birth outcomes. [ABSTRACT FROM AUTHOR]