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 confoun ding has never been carefully assessed. The authors used data from the Coll aborative Perinatal Project, a large prospective cohort study in 12 hospita ls 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 pr essure during pregnancy, the higher the incidence of very premature birth ( <34 weeks) and severe small for gestational age (<5th percentile) in a cons istent dose-response pattern. However, women with low blood pressure were g enerally younger, shorter, lighter, leaner, poorer, and more often a minori ty, and they gained less weight. After the authors controlled for these fac tors, low blood pressure was not associated with preterm birth (adjusted re lative 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 b etween low blood pressure during pregnancy and poor perinatal outcomes is l argely due to confounding by other risk factors. Low blood pressure by itse lf does not increase risk of poor perinatal outcomes at a population level. However, this conclusion may not apply to individual patients who also hav e a compromised plasma volume expansion or pathologic homeostasis.