Children born to mothers who smoke while pregnant are at higher risk of having non-inherited defects such as clubfoot, missing or deformed limbs and facial disorders, according to the first ever comprehensive systematic review to establish which specific defects are linked to smoking in pregnancy. Led by scientists from University College London (UCL) in the UK, the review authors concluded that public health messages should now encourage more women to quit before or during pregnancy.

You can read a report on the review in the 11 July online issue of Human Reproduction Update.

Despite public health warnings that smoking while pregnant can increase the risk of miscarriage and babies being born prematurely, many women still smoke during pregnancy, write the authors. For instance in England and Wales, 17% of women smoke in pregnancy, while in the US that figure is a bit lower at 14%.

This overall figure varies considerably with age, education and across professions. In the UK, 45% of women under the age of 20 smoke during pregnancy, a figure that lead author Professor Allan Hackshaw, of the UCL Cancer Institute, describes as “staggeringly high”. This compares with 9% of women over the age of 35 smoking during pregnancy. 7% of women in occupations classed as managerial or professional smoke during pregnancy in the UK compared to 29% of those in routine or manual jobs.

For their systematic review and meta-analysis, Hackshaw, who is also on the Tobacco Advisory Group of the Royal College of Physicians,where he is a member, and colleagues, searched the Medline database for observational studies published between 1959 and 2010 and picked them out if they reported the Odds Ratio (OR) for babies being born with non-chromosomal birth defects to women who smoked during pregnancy compared with non-smokers.

They found 172 reports that met their criteria, and these gave data on a total of 173,687 babies with malformations and 11,674 332 unaffected controls.

When they analyzed the pooled data they found that smoking in pregnancy raised the risk of babies being born with defects as follows:

  • 33% raised risk of baby being born with skull defects.
  • 28% raised risk of being born with clubfoot, 28% for cleft lip/palate, 27% for gastrointestinal defects, and 26% for missing or deformed limbs.
  • However, the greatest increased risk was for gastroschisis, a defect where parts of the stomach or gut protrude through the skin of the abdomen.

The authors concluded that smoking in pregnancy is an important risk factor for many major birth defects, and:

“These specific defects should be included in public health educational information to encourage more women to quit smoking before or early on in pregnancy, and to particularly target younger women and those from lower socio-economic groups, in which smoking prevalence is greatest.”

Hackshaw said in a statement:

“Maternal smoking during pregnancy is a well established risk factor for miscarriage, low birthweight 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 – this is largely because of past uncertainty over which ones are directly linked.”

But now, thanks to the results of this study, we can “be more explicit about the kinds of serious defects such as deformed limbs, and facial and gastrointestinal malformations,” and he urged women to quit smoking before they get pregnant, or very early in pregnancy so as “to reduce the chance of having a baby with a serious and lifelong physical defect”.

“Maternal smoking in pregnancy and birth defects: a systematic review based on 173 687 malformed cases and 11.7 million controls.”
Allan Hackshaw, Charles Rodeck, and Sadie Boniface.
Hum. Reprod. Update (2011)
First published online: July 11, 2011; doi: 10.1093/humupd/dmr022
Link to Article.

Written by: Catharine Paddock, PhD