BACKGROUND: Cigarette smoking in pregnancy is associated with a significant risk of stillbirth. This risk is dose dependent, and heavier smokers in pregnancy are most susceptible. Uteroplacental insufficiency with growth restriction and placental abruption are the main pathological findings, primarily suggestive of a placental insult. Stopping smoking, particularly pre-pregnancy, reduces this risk of stillbirth. The number of women who continue to smoke in pregnancy is declining. This study examined patterns of stillbirth in pregnant smokers and correlated this with smoking prevalence over the study period.
METHODS: This is a retrospective case-series of all normally formed stillbirths delivered over sixteen years at a large tertiary referral hospital, delivering over 8000 women annually. Logistic regression was used to model smoking rates over time and weighted linear regression was used to compare against the background smoking prevalence.
RESULTS: From 2000 to 2016, 139532 women delivered infants weighing ≥500g at 24 weeks gestation or over. There were 437 normally formed stillbirths during this period, of whom 87 occurred in pregnancies affected by maternal smoking. The overall incidence of stillbirth declined over this period of time, from 6.6/1000 in 2000 to 2.5/1000 in 2016. The prevalence of maternal smoking fell from 28% of mothers in 2000 to 10%. The rates of smoking-associated stillbirth did not show any linear trend (p=0.880). The commonest cause of stillbirth in smokers was placental abruption. Smokers had a significantly higher rate of stillbirth by placental abruption that non-smokers, odds ratio 1.73 (1.01-2.96), p=0.04. The commonest cause of stillbirth in non-smokers was FGR.
CONCLUSIONS: Stillbirth incidence has decreased overall, however the proportion of stillbirths occurring in smokers remains unchanged despite a steady decline in the prevalence of maternal cigarette smoking. These findings suggest that interventions to reduce stillbirth have not altered outcomes in smokers. Women who continue to smoke in pregnancy remain at high risk.