Maternal smoking and gestational hypertension: Heterogeneous effect by timing of the exposure

The objective of this study was to examine whether the association between maternal smoking and gestational hypertension varies by the timing of exposure.

Retrospective cohort study of women identified in 2015 US natality records for singleton births.

Our outcome was whether a woman was diagnosed with gestational hypertension (GH) on the birth record, a category which includes preeclampsia.

Women who smoked before and during pregnancy had a reduced risk for GH relative to non-smokers (adjusted RR 0.92, 95% CI 0.90-0.94). In contrast, women who apparently quit just before the start of pregnancy had higher risk than non-smokers (adjusted RR 1.02, 95% CI 1.00-1.05). When the trimester-specific effects were examined, only women who smoked before pregnancy and in all three trimesters had reduced risk for GH. Smoking mothers who quit just before the start of the 3rd trimester had an increased risk for GH compared to non-smokers (adjusted RR 1.08, 95% CI 1.02-1.16).

In our analysis, women who smoked before pregnancy and in all three trimesters have reduced risk of GH compared to non-smokers, while smokers who reported quitting before pregnancy were at an increased risk. Our results offer new insights into the importance of timing of smoking in pregnancy on risk of GH, and challenge the notion that any smoking during pregnancy has a protective effect.