Quitting Smoking When Newly Pregnant Same as Never Smoking

Kate Johnson

July 08, 2011

July 8, 2011 ( UPDATED July 18, 2011 ) — Women with a newly confirmed pregnancy now have extra incentive to quit smoking, according to the results of the largest study to date looking at the effect of smoking on pregnancy outcomes.

"There's now a second clear message," said senior investigator Nick Macklon, MD, professor of Obstetrics and Gynaecology at the University of Southampton and director of the Complete Fertility Centre, also in Southampton, United Kingdom.

"Not only that smoking is bad for pregnancy, but...women who take the effort to stop even as late as when their pregnancy has been confirmed can achieve birth weights which are the same as if they had never smoked."

Speaking at the 27th Annual Meeting of the European Society of Human Reproduction and Embryology, Dr. Macklon outlined data collected from more than 50,000 pregnancies registered at the Southampton University Medical Centre between 2002 and 2010.

Pregnant women were divided according to smoking status: nonsmokers, those who had stopped more than a year before conceiving, those who had stopped less than a year before conceiving, those who had stopped at the time that the pregnancy was confirmed, and those who continued to smoke up to 10 cigarettes a day, between 10 and 20 cigarettes a day, and more than 20 cigarettes a day.

Comparing babies' birth weights, head circumference, and gestational age between the groups, univariate analysis showed a dose-dependent effect of smoking.

Birth Weights Similar for Quitters

For infants of nonsmokers, the mean birth weight was 3.43 kg. However, for babies of mothers who had quit smoking, birth weights were similar, regardless of whether mothers had quit more than a year ago, less than a year ago, or only when the pregnancy had been confirmed (3.50 kg, 3.45 kg, and 3.45 kg, respectively).

In contrast, infants born to mothers who smoked during pregnancy had lower birth weights: 3.24 kg in those whose mothers smoked up to 10 cigarettes a day and 3.17 kg and 3.11 kg, respectively, in those whose mothers smoked between 10 and 20 or more than 20 cigarettes a day.

A similar effect was noted with head circumference and gestational age, Dr. Macklon told Medscape Medical News.

"Interestingly, the impact of smoking on head circumference was less than on birth weight, suggesting that a 'brain-sparing effect' may protect the brain in the uterus," he commented. "This phenomenon is already well described when babies are found to be too small in the uterus as a result of the placenta not working well.

"A number of small studies have suggested that stopping smoking might prevent extreme effects on growth. However ours is by far the largest yet and allows us to look much more closely at dose effects," continued Dr. Macklon. "Moreover, we show the benefit of stopping smoking on mean birth weight to be far greater than previously reported."

He suggested the findings may help physicians deliver a positive, rather than negative, public health message about smoking in pregnancy, which might be more motivating for women who are trying to quit.

"The campaigns we've been running have had little effect on behavior until now. I would hope this sort of data should encourage us to redouble our efforts."

However, Dr. Macklon was quick to emphasize that these results should not make smokers complacent about quitting before they become pregnant.

"Smoking has a major effect on fertility. Smoking is a contraceptive — it reduces in vitro fertilization success rates by about half — so [smokers] may have to wait a lot longer to conceive. But there are many women who conceive spontaneously despite smoking, and the message should be it's not too late to do something about it."

Birth Defects Still a Risk

According to Margaret A. Honein, PhD, MPH, epidemiologist and chief of the Birth Defects Branch in the National Center on Birth Defects and Developmental Disabilities at the US Centers for Disease Control and Prevention, although the study shows similar outcomes for birth weight, head circumference, and gestational age between babies whose mothers never smoked and those whose mothers quit early, there are other effects of smoking in early pregnancy, "such as an increased risk for a baby to be born with an orofacial cleft or certain heart defects."

"In the [United States], where approximately 50% of pregnancies are unplanned, it is likely that many women don't recognize their pregnancy at the earliest stage — around 4 weeks after the last menstrual period," she told Medscape Medical News.

"If women smoke during this time (4 - 8 weeks into pregnancy), but before they realize they are pregnant, the developing embryo is still being exposed to the various compounds contained in tobacco smoke for many weeks...including nicotine, carbon monoxide, and heavy metals. Exposure to these chemical compounds early in pregnancy, but before a woman realizes she is pregnant, might increase the risk for certain birth defects."

Indeed, the risk for birth defects is often not emphasized when warning women about the risks of smoking in pregnancy, said Allan Hackshaw, MSc, deputy director, Cancer Research UK and University College London Cancer Institute and Cancer Trials Centre, United Kingdom. Mr. Hackshaw is also a member of the Royal College of Physicians Tobacco Advisory Group.

"Maternal smoking during pregnancy is a well-established risk factor for miscarriage, low birth weight, and premature birth. However, very few public health educational policies mention birth defects when referring to smoking, and those that do are not very specific," said Mr. Hackshaw, who is lead author of the first comprehensive review to associate specific birth defects with smoking.

The study, published online July 11 in Human Reproduction Update, analyzed 172 research papers published over the last 50 years, which looked at a combined total of 174,000 cases of malformation compared with 11.7 million control patients.

The study found that smoking in pregnancy was associated with a 26% increased risk for missing or deformed limbs, 28% risk for clubfoot, 27% risk for gastrointestinal defects, 33% risk for skull defects, 25% risk for eye defects, 28% risk for cleft lip/palate, and 50% risk for gastroschisis.

Most of the defects would probably have occurred by 10 to 12 weeks of pregnancy, with limbs and heart problems affected earliest, Mr. Hackshaw told Medscape Medical News.

"I would think that smoking until the pregnancy is diagnosed and then quitting probably carries a higher risk of birth defects than quitting before becoming pregnant, but this might not be easy to reliably determine without large studies," he said.

Dr. Macklon has received research grants, speakers fees and consultancy fees from Ferring, MSD, Merck Serono and Anecova. He is also a shareholder in Complete Fertility Services Ltd. Neither Mr. Hackshaw nor Dr. Honein have disclosed relevant financial relationships.

European Society of Human Reproduction and Embryology 27th Annual Meeting: Abstract O-235. Presented July 6, 2011.

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