Strengths and Limitations
The large population improves precision and allows analyses not
conducted in most prior studies, including examining labetalol
separately from other β-blockers and directly comparing antihypertensive
medications. We studied a diverse population in community practice and
adjusted for many covariates including BP. Limitations include the
potential for residual confounding because treatment was not randomized.
Because we studied medication use in real world clinical practice, there
were not uniform criteria for initiating or intensifying
antihypertensive medications. It is possible that women filled
medications but did not take them, leading to misclassification of
exposure status. All women had health insurance and access to care and
in general, their hypertension was well controlled at the time of the
index fill, which may affect generalizability. Our data did not allow us
to distinguish between spontaneous and indicated preterm birth, which on
average would be expected to bias findings toward the null. The subgroup
of women with gestational hypertension was too small to analyze
separately. We did not have information about use of low dose aspirin,
which the US Preventive Services Task Force recommended for women with
chronic hypertension in 2014.36 The mean difference in
birthweight between medications was small, and it could be argued that a
difference this small is not clinically important. While this may be
true for infants in the normal range, even a small shift of the
birthweight curve to the left could result in a large relative increase
in infants born SGA or with low birth weight, which may have important
consequences for the long term health of these infants.