Effect of maternal smoking on placental weight and placental-to-birth weight ratio in full-term singleton births: a birth registry-based study

BACKGROUND: Maternal smoking during pregnancy is a significant risk factor for adverse pregnancy outcomes. However, studies investigating the association between smoking and placental morphology are limited.

AIM: The study aimed to examine the association between maternal smoking during pregnancy and placental weight, as well as the placental-to-birth weight ratio.

METHODS: A retrospective cohort study was conducted using the data from the Murmansk County Birth Registry. The analysis included singleton births at ≥37 completed weeks of gestation. Multinomial logistic regression was applied to assess the associations between standardized placental weight categories (low, medium, high) and the placental-to-birth weight ratio, and smoking before and during pregnancy, including the number of cigarettes smoked daily.

RESULTS: The mean placental weight was 534.1 grams for male newborns and 523.7 grams for females. Compared with women who never smoked, those who smoked during pregnancy and those who smoked before pregnancy had significantly lower odds of low standardized placental weight. The adjusted relative risk ratio (RRR)=0.75 (95% CI: 0.70–0.81) for smokers, and 0.86 (95% CI: 0.76–0.97) for those who smoked before pregnancy. The likelihood of a higher standardized placental weight was significantly greater among those who smoked during pregnancy and those who had quit smoking before the first antenatal visit: adjusted RRR=1.35 (95% CI: 1.25–1.45) for smokers and 1.21 (95% CI: 1.09–1.36) for those who quit before pregnancy, compared with non-smokers.

The women who smoked before or during pregnancy were less likely to have a low placental-to-birth weight ratio compared with never-smokers (adjusted RRR=0.76 [95% CI: 0.70–0.83] for smokers; adjusted RRR=0.87 [95% CI: 0.77–0.97] for those who quit), and were more likely to have a high standardized ratio (adjusted RRR=1.52 [95% CI: 1.43–1.63] for smokers; adjusted RRR=1.18 [95% CI: 1.06–1.31] for those who quit). An inverse relationship was identified between the number of cigarettes smoked during pregnancy and the likelihood of obtaining higher standardized values for both placental weight and placental-to-birth weight ratio.

CONCLUSION: The study revealed that smoking during pregnancy and quitting prior to the first antenatal visit were associated with higher placental weight and placental-to-birth weight ratio, with a dose-dependent effect observed among smokers. These findings suggest that not only smoking cessation but also reduction in daily cigarette consumption may lower the risk of adverse fetal outcomes, which may serve as a potential motivational tool for promoting primary prevention strategies aimed at reducing adverse pregnancy outcomes among women who smoke.