Does low maternal blood pressure during pregnancy increase the risk of perinatal death?

Background: A recent report described an association between low maximum diastolic blood pressure (DBP) during pregnancy and perinatal death (stillbirth and death in the first week combined). The authors did not account for gestational length, a strong predictor of perinatal death. Methods: We studied 41,089 singleton pregnancies from the U.S. Collaborative Perinatal Project (1959-1966). Results: We observed an association between low maximum DBP and elevated risk of perinatal death. However, this association disappeared after accounting for reverse causation related to gestational length. At any given gestational week, women whose offspring ultimately experienced perinatal death did not have significantly lower maximum DBP than women whose offspring survived the perinatal period. When accounting for the trend of increasing DBP during late pregnancy through gestational-age-specific DBP standardized score, we saw no association between low diastolic blood pressure and perinatal death. Conclusions: Low maximum maternal DBP during pregnancy is a post hoc correlate of perinatal death, not a true risk factor.