Smoking cessation in pregnant women is associated with a reduced risk of preterm birth, a large study from the United States has shown.

Information was obtained from live birth certificates issued between 2011 and 2017 as part of the US National Vital Statistics System. Overall, 25,233,503 pregnant women who delivered live neonates and had data recorded on pre-pregnancy and trimester-specific cigarette smoking frequency were included in the analysis.

Smoking frequency was categorised as 0, 1-9, 10-19, and ≥ 20 cigarettes per day three months prior to pregnancy and in the first, second, or third trimesters.

The modal age at delivery was 25-29 years, with 52.9% of study subjects non-Hispanic white, 23.6% Hispanic and 14.2% non-Hispanic black women. A quarter of the women were high school graduates, 60% had some college education, 60% were married and 33% received social benefits.

The proportion of women who smoked during the three months prior to pregnancy was 10.4%. Only around 1 in 4 women who smoked before pregnancy quit smoking throughout pregnancy.

An increase in the proportion of preterm births with smoking frequency was found in each trimester. For example, in the first trimester, the proportion was 14.4% for those who smoked 1-9 cigarettes per day, 14.8% for those who smoked 10-19 cigarettes per day, and 16.0% for those who smoked ≥ 20 cigarettes per day. In third trimester smokers, the probability was 14.5% for those who smoked 1-9 cigarettes per day, 15.0% for those who smoked 10-19 cigarettes per day, and 16.6% for those who smoked  ≥ 20 cigarettes per day.

The probability of preterm birth decreased the earlier smoking cessation occurred in pregnancy.

For example, the probability of preterm birth was 9.8% (95% CI: 9.7-10.0%) in 25-29 year-old non-Hispanic white primigravida who smoked 1 to 9 cigarettes per day before pregnancy and maintained this frequency throughout their pregnancy.

The probability of preterm birth was 9.0% (95% CI: 8.8-9.1%) if smoking cessation occurred at the start of the second trimester (an 8.9% relative decrease), and 7.8% (95% CI: 7.7-8.0%) if cessation occurred at the start of pregnancy (a 20.3% relative decrease).

The study authors conclude that  smoking cessation may be especially difficult for pregnant women. However, quitting – and quitting early in pregnancy – was associated with reduced risk of preterm birth.

 Dr Peter Saul