Severe Adverse Effects of Smoking May Be Reversible if Mothers Quit Early in Pregnancy

Laurie Barclay, MD

April 04, 2009

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April 3, 2009 — Severe adverse effects of smoking on the fetus may be reversible if mothers quit smoking early in pregnancy, according to the results of a prospective cohort study reported online first March 27 in the British Medical Journal.

"Although stopping smoking in pregnancy has been known for some time to reduce the rate of low birthweight infants and total preterm births, very few studies have addressed whether there is a critical gestation time by which smoking must stop to prevent smoking induced complications of pregnancy," write Lesley M.E. McCowan, MD, from the University of Auckland in New Zealand, and colleagues from the Screening for Pregnancy Endpoints (SCOPE) consortium. "Surprisingly, no studies to date have specifically addressed whether stopping smoking in early pregnancy reduces the rate of spontaneous preterm birth or small for gestational age infants. If a critical period of gestation was identified by which smoking needed to cease to reduce these severe complications, such data would have important public health implications for pregnant women and their maternity care providers."

The goal of this study was to compare pregnancy outcomes for women who quit smoking in early pregnancy vs those who either did not smoke in pregnancy or who continued to smoke.

In Auckland, New Zealand, and Adelaide, Australia, 2504 nulliparous women enrolled in the SCOPE study were grouped by maternal smoking status at 15 (±1) weeks' gestation. The primary study endpoints were spontaneous preterm birth and small-for-gestational-age infants, defined as birth weight less than the tenth customized centile. Using logistic regression, with adjustment for demographic and clinical risk factors, the investigators compared the odds of these endpoints occurring in stopped smokers vs nonsmokers and in current smokers vs stopped smokers.

Smoking status was nonsmoking in 80% of participants (n = 1992), having stopped smoking in 10% of participants (n = 261), and current smoking in 10% of participants (n = 251). For nonsmokers vs stopped smokers, there were no differences in rates of spontaneous preterm birth (4% [n = 88] vs 4% [n = 10]; adjusted odds ratio [OR], 1.03; 95% confidence interval [CI], 0.49 – 2.18; P = .66) or small-for-gestational-age infants (10% [n = 195] vs 10% [n = 27]; OR, 1.06; 95% CI, 0.67 – 1.68; P = .8).

For current smokers vs stopped smokers, there were higher rates of spontaneous preterm birth (10% [n = 25] vs 4% [n = 10]; OR, 3.21; 95% CI, 1.42 – 7.23; P = .006) and of small-for-gestational-age infants (17% [n = 42] vs 10% [n = 27]; OR, 1.76; 95% CI, 1.03 – 3.02; P = .03).

"In women who stopped smoking before 15 weeks' gestation, rates of spontaneous preterm birth and small for gestational age infants did not differ from those in non-smokers, indicating that these severe adverse effects of smoking may be reversible if smoking is stopped early in pregnancy," the study authors write. "Maternity care providers should strive to assist pregnant women who smoke to stop early in pregnancy, emphasizing the major health benefits if they cease to smoke before 15 weeks' gestation."

Limitations of this study include self-reported smoking status not validated by measurement of cotinine levels.

"Our data should not be misinterpreted as a justification to reduce efforts to assist pregnant women to strive to become smoke free if they still smoke after 16 weeks' gestation," the study authors conclude. "Improved pregnancy outcomes have been reported in women who stop by as late as 32 weeks' gestation. Moreover, stopping smoking at any gestation in pregnancy, if sustained in the postpartum period, has enormous additional benefits on newborn and child health."

The New Enterprise Research Fund, Foundation for Research Science and Technology; Health Research Council; and Evelyn Bond Fund, Auckland District Health Board Charitable Trust, supported the New Zealand SCOPE Study. Premier's Science and Research Fund, South Australian Government, supported the Australian SCOPE Study. The study authors have disclosed no relevant financial relationships.

BMJ. Published online March 27, 2009.


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