National trends of pre-eclampsia among women who smoke during pregnancy and their combined effect on maternal-fetal outcomes, United States, 2001-11

Objective: To examine temporal trends of pre-eclampsia (PE) among women who smoke during pregnancy and understand the joint effect of PE and smoking on maternal-fetal outcomes.

Study Design: Cross-sectional analysis of discharge records (2001-2011) from the Nationwide Inpatient Sample. Using International Classification of Diseases, 9^th Edition, Clinical Modification (ICD-9-CM) codes, maternal delivery records were grouped into four exposure categories based on their smoking status and PE diagnosis (neither, smoking only, PE only, and both). ICD-9-CM codes were also used to identify outcomes and important clinical confounders. Multivariable logistic regression was used to calculate adjusted odds ratios (AOR) representing the association between exposure and each adverse outcome. Joinpoint regression was employed to assess trends. The nature of the interaction between pre-eclampsia and smoking was assessed using interaction contrast ratio (ICR).

Results: We found a 2.79% and 1.95% annual increment in the rate of PE among smokers and non-smokers respectively. We observed an increased relative odds of adverse maternal outcomes ranging in magnitude from 68% (postpartum hemorrhage) to over 17-fold (pulmonary edema), and an increased likelihood of fetal outcomes (poor fetal growth (AOR= 5.49, 95% CI [5.11-5.91]), early onset delivery (AOR= 3.76, 95% CI [3.53-4.01]), and still birth (AOR=1.88, 95% CI [1.61-2.20])) among smokers who developed PE.

Conclusion: We found an additive effect of pre-eclampsia and smoking on the risk of pulmonary edema. Prevention of pre-eclampsia through lifestyle modification and promotion of smoking cessation programs during preconception or early pregnancy could reduce the risk of pulmonary edema.