Maternal smoking increases risk of severe neonatal morbidity, even at low intensity

By Dr. Liji Thomas, MDReviewed by Benedette Cuffari, M.Sc.Aug 23 2024

A recent study published in the Journal of Epidemiology and Community Health explores the association between severe neonatal morbidity (SNM) and maternal smoking before or during pregnancy.

Study: Maternal cigarette smoking before or during pregnancy increases the risk of severe neonatal morbidity after delivery: a nationwide population-based retrospective cohort study. Image Credit: Africa Studio / Shutterstock.com

How cigarette smoking affects infants

Newborn death rates have declined significantly over the last century; however, SNM remains a major public health issue. Between 2007 and 2012, a significant rise in admissions to the neonatal intensive care unit (NICU) from 64 to 78 for every 1,000 live births, respectively, was observed in the United States.

SNM increases the risk for prolonged illness and neurodevelopmental issues in affected newborns that may persist into childhood and adulthood. Therefore, it is crucial to identify modifiable risk factors for SNM.

About 10% of pregnant American women smoke, despite the well-documented risks associated with smoking on the developing fetus, some of which include preterm birth, low birth weight, and intrauterine growth restriction (IUGR). Several studies also suggest that SNM is higher among babies born to mothers who smoke.

It remains unclear how the timing and intensity of smoking during or before pregnancy affect neonatal health. Despite this, many women think they can protect their babies adequately by quitting cigarettes when they first find out they are pregnant, whereas others may feel that smoking a few of cigarettes is unlikely to cause harm to the baby.

About the study

The current study included data from the 2016-2019 U.S. National Vital Statistics System. Birth certificate data of 12,150,535 women were used.

This information was compared with the self-reported number of cigarettes smoked every day before pregnancy and during each trimester of pregnancy. Smoking intensity was stratified to obtain a granular risk estimate.

SNM was defined as the need for assisted ventilation immediately after birth, assisted ventilation for more than six hours, NICU admission for continuous mechanical ventilation, surfactant replacement therapy, suspected neonatal sepsis, seizure, or serious neurologic dysfunction. A composite infant SNM was used as the outcome measure.

What did the study show?

Mothers who smoked before or during pregnancy were more likely to give birth to babies with SNM. This increased risk persisted even if the mother smoked one or two cigarettes each day, which is considered very low intensity smoking.

Composite SNM for babies born to mothers who smoked one to two, three to five, six to nine, 10-19, or over 20 cigarettes every day rose by 16%, 22%, 26%, 27%, and 31% respectively, as compared to those who did not smoke before pregnancy. If only NICU admission risk was compared, this risk was increased by 13%, 19%, 22%, 24%, and 29%, respectively.

As compared to never-smokers, even women who smoked before but not during pregnancy had a 12% higher risk of composite SNM. Pregnant women who smoked only during the first, second, or third trimester had a 23%, 40%, and 21% higher SNM risk as compared to never-smokers, respectively.

The risk of composite SNM was 27% higher among babies born to women who smoked before pregnancy. When stratified by trimester, smoking was associated with an approximately 31% increased risk of SNM during each trimester. For NICU admission, the risk was 24%, 32%, and 31% higher in the first, second, and third trimester, respectively, among babies of mothers who smoked.

Conclusions

The study findings confirm the increased risk of SNM among mothers who smoked one to two cigarettes every day before or during pregnancy. Thus, health education in pregnancy should focus on the adverse effects of even low intensity smoking on neonatal health.

The increased risk of NICU admission may be due to the known association between IUGR and maternal smoking. Neurological sequelae like seizures may arise due to alterations in brain structure during fetal development, such as reduced cortical thickness in certain parts of the brain and increased frontal brain volume.

Maternal cigarette smoking may also weaken maternal immune responses or the functions of the placenta, thereby increasing the risk of neonatal sepsis. The current study also reports for the first time that smoking could increase the risk of assisted ventilation for over six hours or surfactant therapy.

There is no safe period, nor any safe intensity, for cigarette smoking in relation to pregnancy.

Women of reproductive age should abstain from cigarette smoking in relation to SNM, acknowledging that smoking is also detrimental from the perspective of other health outcomes.”