The finding that chronic hypertension appeared to modify
the effect of BMI on the risk of PIH is interesting. The effect
may be due to the strength of the association of chronic hypertension
with the outcome in comparison to that of BMI,
although it is unlikely given the size of the population and the
fact that the point estimates were so close to 1. This lack of
association with BMI was noted in a prospective study of 861
women with chronic hypertension.29 It could also reflect a
systematic bias in the diagnosis of PIH by providers when a
woman has underlying chronic hypertension. Alternatively, it
may reflect a shared vascular pathway for the development of
chronic hypertension and PIH, consistent with biological
studies, as well as the established increased risk of chronic
hypertension among women with prior PIH. The outcomes of
women with PIH with and without hypertension would be
important to explore further.
There are several important limitations to this study. Our
data are derived from clinical records, and we rely on data
collected at the point of admission by trained nurses, which is
subject to recall biases as well as incomplete data. In addition,
we are not able to assess the severity of preeclampsia because
of the unavailability of confirmatory laboratory testing. We
think it is not likely that this had an important impact on our
findings, as an earlier study of risk factor patterns found the
ORs associated with each risk factor were of similar magnitude
for both gestational hypertension and preeclampsia.30 In
addition, the incidence of PIH we found at the institution is
within the expected range based on prior epidemiological
studies.2