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Evidence summaries

Changes in Smoking during Pregnancy and Risks of Adverse Birth Outcomes and Childhood Overweight

Smoking during pregnancy is associated with preterm birth, small for gestational age and low birth weight, and with childhood overweight. Quitting smoking before or early in pregnancy reduces the risk. Reducing smoking has limited beneficial effects. Level of evidence: "A"

The level of evidence is upgraded by clear dose-response gradient.

Smoking cessation before pregnancy or during early first trimester is recommended.

Summary

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.

Outcomes for all interventions for smoking cessation in pregnancy compared to control

OutcomeRelative effect(95% CI)Risk with controlRisk 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 100076 per 1000(66 to 87)9402(18) High
Preterm birth (under 37 weeks)RR 0.93(0.77 to 1.11)72 per 100067 per 1000(55 to 80)9222(19) High
Mean birthweight (g)-The mean birthweight (g) was 0MD 55.60 higher(29.82 higher to 81.38 higher)11 338(26) High
NICU admissionsRR 0.78(0.61 to 0.98)118 per 100092 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 HASH(0x2fcfe80)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.

Clinical comments

Note

Date of latest search: 2020-10-28

    References

    • Chamberlain C, O'Mara-Eves A, Porter J et al. Psychosocial interventions for supporting women to stop smoking in pregnancy. Cochrane Database Syst Rev 2017;(2):CD001055. [PubMed]
    • Philips EM, Santos S, Trasande L et al. Changes in parental smoking during pregnancy and risks of adverse birth outcomes and childhood overweight in Europe and North America: An individual participant data meta-analysis of 229,000 singleton births. PLoS Med 2020;17(8):e1003182. [PubMed]
    • Liu B, Xu G, Sun Y et al. Maternal cigarette smoking before and during pregnancy and the risk of preterm birth: A dose-response analysis of 25 million mother-infant pairs. PLoS Med 2020;17(8):e1003158. [PubMed]

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