Placental weight and birthweight: the relations with number of daily cigarettes and smoking cessation in pregnancy. A population study

Abstract

Background: We studied associations of number of daily cigarettes in the first trimester with placental weight and birthweight in women who smoked throughout pregnancy, and in women who stopped smoking after the first trimester.

Methods: We included all women with delivery of a singleton in Norway (n = 698 891) during 1999-2014, by using data from the Medical Birth Registry of Norway. We assessed dose-response associations by applying linear regression with restricted cubic splines.

Results: In total, 12.6% smoked daily in the first trimester, and 3.7% stopped daily smoking. In women who smoked throughout pregnancy, placental weight and birthweight decreased by number of cigarettes; however, above 11-12 cigarettes we estimated no further decrease (Pnon-linearity < 0.001). Maximum decrease in placental weight in smokers compared with non-smokers was 18.2 g [95% confidence interval (CI): 16.6 to 19.7], and for birthweight the maximum decrease was 261.9 g (95% CI: 256.1 to 267.7). In women who stopped smoking, placental weight was higher than in non-smokers and increased by number of cigarettes to a maximum of 16.2 g (95% CI: 9.9 to 22.6). Birthweight was similar in women who stopped smoking and non-smokers, and we found no change by number of cigarettes (Pnon-linearity < 0.001).

Conclusions: In women who smoked throughout pregnancy, placental weight and birthweight decreased non-linearly by number of cigarettes in the first trimester. In women who stopped smoking, placental weight was higher than in non-smokers and increased linearly by number of cigarettes; birthweight was almost similar to that of non-smokers.

Figure 1.

Flow chart of study sample.

Figure 2.

Estimated differences in placental weight, birthweight and placental to birthweight ratio according to number of daily cigarettes in the first trimester of pregnancy. Adjusted for year of delivery. (A-C) Non-smokers and daily smokers throughout pregnancy were included in the analyses (n = 660 154); (D-F) Non-smokers and women who stopped daily smoking after the first trimester were included in the analyses (n = 624 165). Non-smokers were used as the reference category. Data were fitted by the use of a linear regression model with restricted cubic splines with three knots (3, 7 and 15 daily cigarettes). Dashed lines represent 95% confidence intervals.

Figure 3.

Estimated differences in placental weight z-scores and birthweight z-scores according to number of daily cigarettes in the first trimester of pregnancy. Adjusted for year of delivery. (A, B) Non-smokers and daily smokers throughout pregnancy were included in the analyses (n = 660 154). (C, D) Non-smokers and daily smokers who stopped after the first trimester (n = 624 165) were included in the analyses. Non-smokers were used as the reference category. Data were fitted by the use of a linear regression model with restricted cubic splines with three knots (3, 7 and 15 daily cigarettes). Dashed lines represent 95% confidence intervals.