Limited evidence suggests that women with a history of preterm delivery are at increased risk for developing hypertension, but it isn't clear whether this association is attributable to confounding by genetic or environmental factors. In a large national cohort study in Sweden, researchers examined the long-term risks of chronic hypertension associated with preterm delivery and assessed potential confounding by shared genetic or environmental factors within families.

In 46.1 million person-years of follow-up, 351,189 of 2,195,989 women (16%) who had singleton deliveries were diagnosed with hypertension.

Shorter pregnancy duration was associated with significantly higher future risks of chronic hypertension, even after adjustment for preeclampsia, other hypertensive disorders of pregnancy, and other potential confounders. Within 10 years after delivery, the adjusted hazard ratio for hypertension associated with preterm delivery was 1.67; when further stratified, it was 2.23 for extremely preterm, 1.85 for moderately preterm, 1.55 for late preterm, and 1.26 for early-term delivery. Each additional week of pregnancy was associated with a mean 7% lower risk. The risk of hypertension decreased over time but remained significantly elevated up to 43 years after delivery. Co-sibling analyses indicated these findings were only partly explained by shared familial factors.

One limitation of the study was that detailed clinical records weren't available to verify hypertension diagnoses. Also, current diagnostic criteria for hypertension have lower blood pressure cutoffs, which would result in a higher prevalence of hypertension than reported in this study.