Low blood pressure during pregnancy and poor perinatal outcomes: an obstetric paradox.

Low blood pressure during pregnancy has been associated with poor perinatal outcomes. However, whether this association is causal or is due to confounding has never been carefully assessed. The authors used data from the Collaborative Perinatal Project, a large prospective cohort study in 12 hospitals in the United States from 1959 to 1966. A total of 28,095 subjects were included. At first glance, it appeared that the lower the baseline blood pressure during pregnancy, the higher the incidence of very premature birth (<34 weeks) and severe small for gestational age (<5th percentile) in a consistent dose-response pattern. However, women with low blood pressure were generally younger, shorter, lighter, leaner, poorer, and more often a minority, and they gained less weight. After the authors controlled for these factors, low blood pressure was not associated with preterm birth (adjusted relative risks ranging from 0.86 to 0.93, p > 0.05) or small for gestational age (relative risks ranging from 0.45 to 2.0). Therefore, the association between low blood pressure during pregnancy and poor perinatal outcomes is largely due to confounding by other risk factors. Low blood pressure by itself does not increase risk of poor perinatal outcomes at a population level. However, this conclusion may not apply to individual patients who also have a compromised plasma volume expansion or pathologic homeostasis.