Women who have had a preterm or early-term birth are at greater risk for an early delivery during a subsequent pregnancy, compared with women whose previous pregnancies have been full term, according to a large cohort study.

“This study confirms that a prior preterm birth increases the risk for a subsequent preterm birth with higher odds among those with a prior early preterm birth. Risks extend to both spontaneous and indicated prior preterm birth,” Juan Yang, PhD, of the California Department of Public Health, and his colleagues wrote (Obstet Gynecol. 2016;128:364-72. doi: 10.1097/AOG.0000000000001506 ).

The researchers retrospectively examined the records of 163,889 singleton live births that occurred in California from 2005 through 2011 whose mothers had experienced two consecutive pregnancies over that time period that had gestation periods of anywhere between 20 and 44 weeks.

Women were placed into cohorts based on the gestation periods of their first and second pregnancies: fewer than 32 weeks (early preterm), 32-36 weeks (preterm), 37-38 weeks (early term), and 39 weeks or more (full term).

Women whose first pregnancy lasted for fewer than 32 weeks’ gestation – an early preterm birth – were at the highest risk of having another delivery at fewer than 32 weeks in their second pregnancy, compared with women with term pregnancies (adjusted odds ratio, 23.3; 95% confidence interval, 17.2-31.7).

Women with a prior early term birth (37-38 weeks) had more than a twofold increased risk for a preterm birth, with an adjusted OR of 2.0 for a subsequent birth before 32 weeks and an adjusted OR of 3.0 for a subsequent birth from 32 to 36 weeks of gestation. The adjusted OR for another early term birth in this group was 2.2.

The researchers wrote that their findings “provide a better understanding of early term birth and the importance of continuation of gestation beyond 38 weeks of gestation,” noting that “although the association of prior early term birth with subsequent preterm birth is less strong than prior preterm birth, many more women fall in this category, making this an important risk factor to the overall societal burden of preterm birth.”

The study was funded by the Preterm Birth Initiative-California; the University of California, San Francisco; the March of Dimes Prematurity Research Centers; the Bill and Melinda Gates Foundation; and the Eunice Kennedy Shriver National Institute of Child Health and Human Development. The researchers reported having no financial disclosures.


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