Women who smoke during pregnancy are at increased risk of preterm birth, according to a study in JAMA Network Open.
“Our goal was to quantify the reduction in the risk of premature birth when expectant mothers who smoke during pregnancy are able to quit,” said principal investigator Samir Soneji, PhD, an associate professor of health policy at Dartmouth College in Hanover, New Hampshire.
The cross-sectional study gleaned information from live birth certificates issued between 2011 and 2017 from U.S. states that implemented the 2003 revision of the U.S. live birth certificate.
Overall, 25,233,503 expectant mothers who delivered live neonates and recorded known prepregnancy and trimester-specific cigarette smoking frequency were analyzed.
The modal age at delivery was 25 to 29 years, with 52.9% of study subjects non-Hispanic white, 23.6% Hispanic and 14.2% non-Hispanic black women.
One-fourth of expectant mothers were high school graduates and 60% had some college education. A total of 60% also were married and 33% received benefits from the Special Supplemental Nutrition Program for Women, Infants and Children (WIC).
Regarding smoking rates, 89.6% of overall study subjects did not smoke during the 3 months prior to pregnancy, compared to 10.4% who did.
Despite the substantial benefits of smoking cessation during pregnancy, the proportion of prepregnancy smokers who quit smoking during pregnancy remained essentially stagnant throughout the study period: 24.3% in 2011 versus 24.5% in 2017.
Similarly, the proportion of prepregnancy smokers who quit during the third trimester remained unchanged: 39.53% in 2011 compared to 39.7% in 2017.
“The lack of progress in smoking cessation during pregnancy over time surprised and concerned us,” Dr. Soneji told Contemporary OB/GYN.
Dr. Soneji noted that higher-frequency smoking is very common among women who smoke during pregnancy; for example, 46.9% of third-trimester smokers smoked at least 10 cigarettes a day in 2017.
On the other hand, the likelihood of preterm birth decreased parallel to earlier smoking cessation during pregnancy — up to a 20% relative decrease if quitting occurred at the beginning of pregnancy.
The probability of preterm birth was 9.8% (95% CI: 9.7% - 10.0%) among 25- to 29-year old non-Hispanic white, primigravida and primiparous expectant mothers who smoked 1 to 9 cigarettes per day prior to pregnancy and maintained that frequency throughout their pregnancy.
However, the probability of preterm birth decreased to 9.0% (95% CI: 8.8% -9.1%) if smoking cessation occurred at the start of the second trimester, and to 7.8% (95% CI: 7.7% - 8.0%) if quitting was at the start of pregnancy.
“Pregnancy is a time when women are frequently in contact with the healthcare system and their healthcare providers,” Dr. Soneji said. “Healthcare providers could devote more attention and provide greater support for smoking cessation therapies among their pregnant patients who smoke.”
Dr. Soneji admitted, though, that smoking cessation is challenging for any adult, given the strong addictive properties of nicotine. “And cessation may be even harder for expectant mothers who smoke because healthcare providers may be reluctant to recommend certain cessation therapies because they are FDA category C drugs,” he said. “Nonetheless, counseling and therapy are available modalities that may prove effective.”
As a follow-up study, Dr. Soneji and his colleagues plan to determine how quit rates and smoking intensity impact the risk of infant mortality. "If we can show that stopping, and stopping early, reduces the risk of infant mortality, this may further motivate mothers to give up smoking, at least during pregnancy,” he said.
Dr. Soneji reports no relevant financial disclosures.