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.