Is any prenatal smoking safe for preterm birth?
Among 25.6 million US births, even 1–2 cigarettes/day in the first or second trimester raised preterm-birth odds; quitting before pregnancy matched nonsmoker risk.
Source
Maternal cigarette smoking before and during pregnancy and the risk of preterm birth: A dose-response analysis of 25 million mother-infant pairs
Study at a glance
- Design
- Cohort — Maternal smoking before and during pregnancy vs preterm birth
- N
- Perinatal cohort; single analytic N not extracted
- Population
- Pregnant women with recorded smoking status before and during pregnancy
- Outcome
- Preterm-birth risk associated with maternal cigarette smoking
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Inspect source passages
Is any prenatal smoking safe for preterm birth?
These passages occur verbatim in the stored paper text. They support the points listed below; they do not verify every statement in the explanation.
25,623,479 women were included (mean age 29).
This study included 25,623,479 women, with a mean age of 29 years (range 20–50 years)
First-trimester 1–2 cigarettes/day OR 1.31 (1.29–1.33) vs nonsmokers.
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 (1.29–1.33), 1.31 (1.30–1.32), 1.33 (1.31–1.35), 1.44 (1.43–1.45), and 1.53 (1.52–1.55)
What they did
Analyzed NVSS singleton mother–infant pairs (2011–2018) with complete smoking and gestational-age data, estimating adjusted odds of preterm birth by smoking status and cigarettes/day before pregnancy and in the first/second trimester.
What they found
First-trimester smoking of 1–2 up to ≥20 cigarettes/day had ORs from 1.31 to 1.53 vs nonsmokers. Low-intensity smoking in either trimester still increased risk. Quitting before pregnancy yielded risk comparable to never smokers; quitting only after conception did not fully normalize risk. Associations varied by age, race/ethnicity, and education.
The limits
What it doesn't show
Self-reported smoking on birth certificates can misclassify exposure; residual confounding and selection after excluding hypertension/diabetes limit causal claims about intervention effects.
Key terms
- Preterm birth
- Delivery before 37 completed weeks of gestation.
- NVSS
- US National Vital Statistics System birth-certificate data.
- Dose–response
- How risk changes as daily cigarette count increases.
- Trimester-specific smoking
- Smoking reported before pregnancy vs first or second trimester.
- Odds ratio
- Relative odds of preterm birth vs nonsmokers after covariate adjustment.
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Quiz yourself
How many mothers were in the analytic cohort?
Common questions
How large was the cohort?
25,623,479 mothers (mean age 29).
Lowest risky intensity?
Even 1–2 cigarettes/day in first or second trimester.
Does quitting before pregnancy help?
Yes — risk matched nonsmokers regardless of prior intensity.
Main public-health takeaway?
No safe level or safe trimester for maternal smoking.
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