Association Between Maternal Smoking, Isolated Proteinuria During Pregnancy and Preterm Birth: A Finnish Registry Analysis.

Introduction: Smoking during pregnancy (SDP) seems to paradoxically decrease the likelihood of preeclampsia. We aimed to investigate the association between smoking and isolated proteinuria during pregnancy. In addition, we investigated the associations and potential interaction between smoking and proteinuria on the risk for preterm birth.

Methods: The study included all women with singleton pregnancies ( = 791,183) in Finland during the years 2006 to 2018, excluding those with previous kidney diseases, gestational hypertension or diabetes, or preeclampsia. Information on smoking and background factors were derived from the Finnish Medical Birth Register. Smoking was categorized as no smoking, quit in the first trimester, or continued smoking thereafter. Information on isolated proteinuria at any time of pregnancy was derived from the Finnish Hospital Discharge Register and the Finnish Medical Birth Register with an International Classification of Diseases (ICD) (Tenth Revision [ICD-10]) code O12, excluding gestational edema (O12.0). Logistic and linear regression models were used to estimate the associations.

Results: Of the participants, 14.6% were smokers, of which 36.9% quit smoking; 2534 (0.3%) had a diagnosis of isolated proteinuria. Those who quit smoking (odds ratio [OR] = 1.31, 95% confidence interval [CI] = 1.14-1.52) and those who continued smoking (OR = 1.29, 95% CI=1.15-1.46) were associated with having a diagnosis of isolated proteinuria. Isolated proteinuria (OR = 1.24, 95% CI = 1.03-1.49) and those who continued smoking (OR = 1.45, 95% CI = 1.40-1.50) were associated with preterm birth. The interaction of smoking and isolated proteinuria with preterm birth lacked statistical significance.

Conclusion: We found evidence of an association between smoking and the diagnosis of isolated proteinuria. Furthermore, smoking and a diagnosis of isolated proteinuria were both associated with a higher risk for preterm birth.