By John C. Hobbins, MD

Professor, Department of Obstetrics and Gynecology, University of Colorado School of Medicine, Aurora

Dr. Hobbins reports no financial relationships relevant to this field of study.

SYNOPSIS: A study using Ohio state birth data showed that women who stopped smoking by the end of the first trimester had the same rates of preterm birth as nonsmokers. Women who stopped in the second trimester had preterm birth rates similar to those who smoked all the way through pregnancy.

SOURCE: Moore E, Blatt K, Chen A, et al. Relationship of trimester-specific smoking patterns and risk of preterm birth. Am J Obstet Gynecol 2016;215:109.e1-6.

Even though fewer people are smoking now than 10 years ago, there still remains a formidable number of individuals who, despite being aware of the dangers of smoking, continue to light up. A previous Alert touched on how smoking fallout from passive exposure to cigarette smoke can have a detrimental effect on non-smoking pregnant women.1

One reason about 11.5% of pregnant women continue to smoke is that it is hard to quit. Yet, as the featured study suggests, doing so can have significant benefit if undertaken early in pregnancy.

In Ohio, the overall rate of smoking in pregnancy is 23%. A group of investigators, having a ready population to study, set out to determine if there was benefit to cessation in early pregnancy with regard to one outcome variable: preterm birth (PTB). They examined the Ohio birth records of 913,757 patients from 2006 through 2012, 25% of whom were smokers. They broke the smoking patients into four groups: 1) early quitters, who only smoked preconception; 2) first trimester quitters; 3) second trimester quitters; and 4) those who smoked throughout pregnancy. Data regarding preterm birth were compared to data from nonsmokers. The authors attempted to correct for confounding influences.

The early quitters had a PTB rate that was no different than nonsmokers; in the first trimester quitters, the PTB rate less than 37 weeks was not significantly higher than nonsmokers but, strangely, it was higher for PTB below 28 weeks (odds ratio [OR], 1.20; 95% confidence interval [CI], 1.03-1.40). Quitting late resulted in the highest rate of PTB prior to 37 weeks, compared with nonsmokers (OR, 1.70; 95% CI, 1.60-1.80), even after accounting for confounding variables. However, the rate was not significantly different than if they smoked throughout pregnancy. The most dramatic finding was that quitting late was associated with the greatest increase in PTB at less than 37 weeks, whatever the cause (65% increase in spontaneous labor and a 78% increase in indicated intervention).


One can conclude from this study that the women who were able to stop smoking by the end of the first trimester had an excellent chance of avoiding cigarette-associated preterm birth. Hopefully, this will provide further evidence during counseling to motivate patients to stop smoking.

The ill effects of smoking are not based on nicotine alone. Smokers have high levels of carboxyhemoglobin, which can affect transfer of oxygen and nutrients across the placenta, giving new meaning to the term “placental barrier.” Quitting late in pregnancy has little effect on the one outcome variable studied here, preterm birth, but is borne out indirectly by the observation that many patients who have stopped smoking at varying intervals after the first trimester often display ultrasound signs of “premature placental senescence” (increased areas of echogenicity) as early as 32 weeks.

From this study it seems that the earlier a woman quits smoking, the smaller her chance of delivering a premature baby. Another study,2 feature in the earlier Alert, has shown that those who do stop early or, interestingly, have been subjected to passive cigarette smoke (as measured by plasma cotinine levels) are at higher risk for pre-eclampsia compared with non-smokers.


  1. Hobbins JC. Passive smoking exposure and preeclampsia. OB/GYN Clinical Alert 2014;31:21-22.
  2. Luo ZC, Julien P, Wei SQ, et al. Plasma cotinine indicates an increased risk of preeclampsia in previous and passive smokers. Am J Obstet Gynecol 2014;210:232.e1-5.