Risk of preterm birth by maternal age at first and second pregnancy and race/ethnicity.

We examined the risk of preterm birth (PTB, <37 weeks' gestation) in a second pregnancy and analyzed the extent to which this risk varies by maternal age and race/ethnicity. The sample included nulligravida mothers in California who delivered two singletons between 2005 and 2011. Logistic regression was used to calculate the odds of PTB in the second pregnancy. Within each race/ethnicity stratum, women delivering term infants in their first pregnancy and between 25 and 34 years old for both pregnancies served as the referent group. There were 2,90,834 women included in the study. Among women who delivered their first infant at term, the odds of delivering their second infant early differed by race and age. Hispanic, Black and Asian non-Hispanic women who were <18 years for both pregnancies were at higher odds of having a PTB in their second pregnancy (adjusted odds ratios 1.7, 3.3 and 2.9, respectively). Asian non-Hispanic women who were <18 years for their first delivery at term and between 18 and 24 years for their second delivery, or were >34 years for both, were also at higher odds of delivering their second baby prematurely (adjusted odds ratios 1.9 and 1.3, respectively). Women who deliver their first infant at <37 weeks of gestation are at 3 to 7 times higher odds of delivering their second infant preterm. Providers should consider including information about these risks in counseling their patients.