The level of evidence is upgraded by clear dose-response gradient.
Smoking cessation before pregnancy or during early first trimester is recommended.
A Cochrane review [Abstract] 1 included 88 studies with over 28 000 subjects assessing psychosocial interventions for smoking cessation. Especially counselling, feedback and incentives were effective for reducing low birthweight and admissions to neonatal intensive care compared with usual care.
| Outcome | Relative effect(95% CI) | Risk with control | Risk with Interventions (95% CI) | №of participants(studies) Quality of evidence |
|---|---|---|---|---|
| Low birthweight (under 2500 g) | RR 0.83(0.72 to 0.94) | 92 per 1000 | 76 per 1000(66 to 87) | 9402(18) High |
| Preterm birth (under 37 weeks) | RR 0.93(0.77 to 1.11) | 72 per 1000 | 67 per 1000(55 to 80) | 9222(19) High |
| Mean birthweight (g) | - | The mean birthweight (g) was 0 | MD 55.60 higher(29.82 higher to 81.38 higher) | 11 338(26) High |
| NICU admissions | RR 0.78(0.61 to 0.98) | 118 per 1000 | 92 per 1000(72 to 116) | 2100(8) High |
An individual participant data meta-analysis 2 assessing changes in smoking during pregnancy and risk of adverse birth outcomes included 229 158 singleton births (28 cohorts in Europe and North America). Compared with nonsmoking mothers, maternal first trimester smoking only was not associated with adverse birth outcomes but was associated with a higher risk of childhood overweight (odds ratio [OR] 1.17, 95% CI 1.02 to1.35, P value = 0.030). Children from mothers who continued smoking during pregnancy had higher risks of preterm birth (OR 1.08, 95% CI 1.02 to 1.15, P value = 0.012), small size for gestational age (OR 2.15, 95% CI 2.07 to 2.23, P value < 0.001), and childhood overweight (OR 1.42, 95% CI 1.35 to1.48, P value < 0.001). Mothers who reduced the number of cigarettes between the first and third trimester, without quitting, still had a higher risk of small size for gestational age. However, the corresponding risk estimates were smaller than for women who continued throughout pregnancy. Reducing the number of cigarettes during pregnancy did not affect the risks of preterm birth and childhood overweight.
A dose-response analysis 3 assessing smoking and the risk of preterm birth included 25 million mother-infant pairs. Maternal smoking during pregnancy was associated with an increased risk of preterm delivery. The adjusted ORs (95% CI) of preterm birth for mothers who smoked 1-2, 3-5, 6-9, 10-19, and ≥20 cigarettes per day during the first trimester compared with mothers who did not smoke were 1.31 (1.29 to 1.33), 1.31 (1.30 to 1.32), 1.33 (1.31 to 1.35), 1.44 (1.43 to 1.45), and 1.53 (1.52 to 1.55), respectively (all P values < 0.001), whereas for those who smoked during the second trimester, the corresponding ORs were 1.37 (1.35 to 1.39), 1.36 (1.35 to 1.38), 1.36 (1.34 to 1.38), 1.48 (1.47 to 1.49), and 1.59 (1.58 to 1.61), respectively (all P values < 0.001). Furthermore, smokers who quit before pregnancy, regardless of smoking intensity, had a comparable risk of preterm birth with nonsmokers, although this was not the case when cessation occurred in the first or second trimester of pregnancy.
Date of latest search: 2020-10-28
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