Maternal cigarette smoking prior to or during pregnancy may be associated with an increased risk of severe neonatal morbidity, even at low levels of smoking, according to a large-scale U.S. study.
In the study, published in the Journal of Epidemiology and Community Health, investigators analyzed the data from 12.2 million mother-infant pairs and found that smoking as little as one to two cigarettes per day was linked to higher rates of severe neonatal morbidity (SNM).
The investigators examined birth certificate data from the U.S. National Vital Statistics System for live singleton births from 2016 to 2019. They found that 9.3% of women reported smoking in the 3 months before pregnancy, while 7.0%, 6.0%, and 5.7% reported smoking in the first, second, and third trimesters, respectively.
The study defined composite SNM as one or more of the following: assisted ventilation immediately following delivery, assisted ventilation for over 6 hours, neonatal intensive care unit (NICU) admission, surfactant replacement therapy, suspected neonatal sepsis, and seizure. Overall, 9.4% of the infants experienced SNM.
Among the key findings were:
- Compared with nonsmokers, women who smoked prior to pregnancy had a 1.27 times higher risk of infant composite SNM (95% confidence interval [CI] = 1.26–1.28).
- Smoking in the first, second, or third trimester was associated with a 1.31 (95% CI = 1.30–1.32), 1.32 (95% CI = 1.31–1.33), and 1.31 (95% CI = 1.30–1.32) times higher risk of composite SNM, respectively.
- Smoking one to two cigarettes per day prior to pregnancy was linked to a 1.16 times higher risk of composite SNM (95% CI = 1.13–1.19).
- A potential dose-response relationship was observed, with the risk of SNM increasing with smoking intensity.
The investigators adjusted for multiple confounders including maternal age, race/ethnicity, education, marital status, prepregnancy body mass index, infant sex, gestational age, parity, prenatal care visits, and pregnancy complications.
Regarding individual SNM components, maternal smoking was significantly associated with a higher risk of each outcome, including:
- NICU admission (odds ratio [OR] = 1.24–1.31 across time periods)
- Assisted ventilation immediately after delivery (OR = 1.25–1.29)
- Suspected neonatal sepsis (OR = 1.30–1.32)
- Seizure or serious neurologic dysfunction (OR = 1.54–1.67).
The investigators also examined smoking cessation patterns. Compared with never-smokers, women who stopped smoking during pregnancy still had an elevated SNM risk; however, the associations were not always statistically significant for some cessation subgroups.
Additional population characteristics revealed that smokers tended to be younger (median age = 27.0 years, interquartile range [IQR] = 23.0–31.0) compared with nonsmokers (median age = 29.0 years, IQR = 25.0–33.0). Among smokers, 75.9% were non-Hispanic White, 11.1% were non-Hispanic Black, and 7.0% were Hispanic. Educational attainment among smokers was lower, with 20.7% having less than a high school education, 41.7% with a high school education, and 37.6% with more than a high school education. Notably, 70.4% of smokers were unmarried.
Detailed smoking patterns showed that among women who smoked prior to pregnancy, 6.7% smoked one to two cigarettes per day, 18.6% smoked three to five per day, 6.9% smoked six to nine per day, 30.7% smoked 10 to 19 per day, and 35.5% smoked 20 or more per day.
The prevalence of individual SNM components varied between nonsmokers and smokers:
- NICU admission: 7.2% for nonsmokers vs 9.8% to 10.7% for smokers across trimesters
- Assisted ventilation immediately after delivery: 3.7% for nonsmokers vs 5.4% to 5.7% for smokers
- Suspected neonatal sepsis: 1.9% for nonsmokers vs 2.7% to 2.8% for smokers.
Subgroup analyses demonstrated that the associations between smoking and SNM were consistent across maternal age groups, race/ethnicity categories, and infant birth years. Sensitivity analyses yielded consistent results after excluding women with eclampsia, gestational hypertension, or diabetes, as well as after excluding women who underwent cesarean section or had preterm births.
The study's findings contrasted with a 2021 Australian study (n = 6,243) that found no association between maternal smoking and composite SNM. However, the results aligned with a 2014 Finnish study (n = 1.2 million) that showed an increased risk of NICU admission with early pregnancy smoking.
The investigators concluded that there appeared to be no safe period or level of cigarette smoking shortly prior to or during pregnancy with regards to SNM risk. These findings added to the substantial body of evidence on the harms of maternal smoking and may help inform clinical practice and public health efforts aimed at reducing tobacco use among women of reproductive age.
The authors declared having no competing interests.