Short interpregnancy interval as a risk factor for preterm birth in non-Hispanic Black and White women in California

OBJECTIVE: Short interpregnancy interval (IPI) is associated with adverse pregnancy outcomes, including preterm birth (PTB < 37 weeks GA). We investigated whether short IPI (< 6 months) contributes to the higher PTB frequency among non-Hispanic Blacks (NHB). STUDY DESIGN: Using a linked birth cohort >1.5 million California live births, we examined frequencies of short IPI between racial/ethnic groups and estimated risks by multivariable logistic regression for spontaneous PTB. We expanded the study to births 1991–2012 and utilized a ‘within-mother’ approach to permit methodologic inquiry about residual confounding. RESULTS: NHB women had higher frequency (7.6%) of short IPI than non-Hispanic White (NHW) women (4.4%). Adjusted odds ratios for PTB and short IPI were 1.64 (95%CI 1.54, 1.76) for NHW and 1.49 (1.34, 1.65) for NHB. Using within-mother analysis did not produce substantially different results. CONCLUSIONS: Short IPI is associated with PTB but does not explain risk disparity between NHWs and NHBs.