BMJ  2007;334:499 (10 March), doi:10.1136/bmj.39146.416343.BE

News

Smoking in pregnancy slows growth of baby's head

Abergavenny

The heads of fetuses of women who smoke during pregnancy grow less than those of fetuses whose mothers don't smoke, a study concludes. But mothers who give up smoking as soon as their pregnancy is confirmed have babies whose head circumference grows as much as those of women who never smoke during pregnancy, it says.

With the help of ultrasound images taken during pregnancy, the researchers found that the circumference of the heads of fetuses of women who smoked throughout pregnancy grew half a millimetre less a week during late pregnancy than those of non-smokers. Femur lengths and abdominal circumferences also increased more slowly in the fetuses of smokers, says the study, which was published online on 28 February in the American Journal of Epidemiology (http://aje.oxfordjournals.org, doi: 10.1093/aje/kwm014).

But the study, which involved 7098 women in the Netherlands, found that growth rates of the fetuses of women who gave up smoking when their pregnancy was confirmed and those of non-smokers were similar.

"Continued maternal smoking in late pregnancy seems to affect all fetal growth characteristics and tissues. Both the timing and the size of the effects suggest that maternal smoking in pregnancy affects first peripheral tissues and then central tissues," wrote the authors, from the Generation R Study Group and Erasmus Medical Centre, Rotterdam.

Although it is known that smoking during pregnancy leads to low birth weight—through reduced supplies of nutrients and oxygen—the authors say that low birth weight is an inappropriate measure of the adverse effects of smoking.

They say that the studies that have looked at the effect of maternal smoking on fetal growth involved only small groups of women or hospital based populations and that the investigators were unable to adjust for potential confounders. Nor did these studies examine the effects of smoking on characteristics of fetal growth during different stages of pregnancy.

The researchers used questionnaires to assess smoking in early, middle, and late pregnancy and repeatedly measured fetal growth characteristics in middle and late pregnancy.

One in four (26%) of the mothers, who were aged 15 to 46 years, reported smoking in early pregnancy, and 17% of the total study group continued smoking after their pregnancy was known. The mean birth weight of the babies was 3454 g in women who did not smoke at all during pregnancy and 3251 g in those who continued smoking after they knew about the pregnancy.

The researchers found that in comparison with fetuses of women who didn't smoke, fetuses of women who continued to smoke for the last six months of pregnancy showed reduced growth in head circumference of 0.56 mm a week (95% confidence interval 0.4 mm to 0.73 mm), in abdominal circumference of 0.58 mm a week (0.34 mm to 0.81 mm), and in femur length of 0.19 mm a week (0.14 mm to 0.23 mm).

The reduced growth began at different stages of pregnancy: that in femur length began in mid-pregnancy (18 to 24 weeks), while the reductions in growth of head and abdominal circumferences began in late pregnancy (25 weeks).

The authors say that the associations were independent of potential confounders. The analysis took into account a number of variables, including the women's body mass index, age, educational level, height, ethnicity, and parity and the sex of the fetuses.

CiteULike    Complore    Connotea    Del.icio.us    Digg    Reddit    Technorati    What's this?

Rapid Responses:

Read all Rapid Responses

Observer Bias?

Ifor G Edwards
bmj.com, 12 Mar 2007 [Full text]