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To compare preterm birth rates and gestational length in four race–nativity groups including Somali Americans.Using a retrospective cohort study design of Ohio birth certificates, we analyzed all singleton births between 2000 and 2015 from four groups of women categorized as U.S.-born, non-Hispanic white (USBW), U.S.-born, non-Hispanic black (USBB), African-born black (ABB, primarily of West African birth country), and Somalia-born (SB). An algorithm trained on maternal names was used to confirm Somali ethnicity. Gestational length was analyzed as completed weeks or aggregated by clinically relevant periods. Risk of spontaneous and health care provider–initiated preterm birth was calculated in a competing risk model.Births to women in the designated groups accounted for 1,960,693 births (USBW n=1,638,219; USBB n=303,028; ABB n=10,966, and SB n=8,480). Women in the SB group had a lower preterm birth rate (5.9%) compared with women in the USBB (13.0%), ABB (8.4%), and USBW (7.9%) groups (P<.001). Women in the SB group had a higher frequency of postterm pregnancy (5.8% vs less than 1%, P<.001 for all groups). The lower rate of preterm birth in the SB group was unrelated to differences in parity or smoking or whether preterm birth was spontaneous or health care provider–initiated. The lower rate of preterm birth and tendency for prolonged gestation was attenuated in ethnic Somali women born outside Somalia.We report a positive disparity in preterm birth and a tendency for prolonged gestation for ethnic Somali women in Ohio. Etiologic studies in multiethnic cohorts aimed to uncover the sociobiological determinants of gestational length may lead to practical approaches to reduce prematurity in the general population.