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BJOG Release: Stop Smoking If You Are Pregnant

September 05, 2017

New research to be published in BJOG: An International Journal of Obstetrics and Gynaecology examines the results of maternal smoking on birth outcomes.

Previous studies have focused on the consequences of smoking during each pregnancy, in isolation. This new study analysed outcomes according to whether women continued to smoke in successive pregnancies, or managed to give up smoking after their first pregnancy. Previous pregnancy outcomes were compared to subsequent pregnancy outcomes in the same sample population.

244, 840 mothers from New South Wales, Australia who had two successive singleton deliveries over the period 1994 - 2004 were studied by Dr Mohammed Mohsin and Professor Bin Jalaludin at Liverpool Hospital in Sydney. The majority of women were between 25 to 34 years old and 87% had antenatal care by the 20th week of pregnancy. The interval between first and second child was between 12 - 24 months in a third of mothers studied.

The proportions of mothers who smoked were 18.7% during the first pregnancy and 17.5% in the second pregnancy. Researchers found that 72.7% of smokers in their first pregnancy continued to smoke in their second pregnancy.

Preterm births were the result in 5.9% of all first and 4.9% of all second deliveries. The findings indicate that mothers who had a previous preterm birth were at an increased risk of a repeat preterm birth in the next pregnancy. Researchers found the risk of having a preterm birth in a subsequent pregnancy was increased for those who carried on smoking and was greatest for heavy smokers.

Low birth weight (LBW) was seen in 5.2% of all first and 3.8% of all second births. Continued smoking in the subsequent pregnancy was associated with the highest rate of LBW infants. Researchers found that if a mother continued to smoke heavily during her second pregnancy, the odds ratio of a low birthweight baby (compared with babies of women who never smoked) was 4, compared with 2.1 if she gave up completely.

Others findings from the study point to the increased risk of poor perinatal outcomes associated with smoking, namely; stillbirths and neonatal deaths. Researchers believe that smoking may increase the risk of intrauterine infections which result in placental complications. Smoking during pregnancy may also stimulate the production of hormones such as prostaglandin E2 which causes the womb to contract, thus resulting in preterm births. In addition, smoking reduces the level of type III collagen, a fibrous structural protein, and this may increase the risk of preterm rupture of the membranes. The results in this study are consistent with previous research on smoking during pregnancy.

Professor Bin Jalaludin from the University of New South Wales and Director at the Centre for Research, Evidence Management and Surveillance at Liverpool Hospital, Sydney, said "We know that women who smoke during pregnancy have smaller babies. This study shows that women who smoke in their first pregnancy but not in their second pregnancy still continue to have a two-fold higher risk of a small baby in the second pregnancy compared to women who do not smoke at all, although the risk is less than for women who continue to smoke, which is four-fold higher.

"This is an important finding and suggests that there appears to be a carry-over effect of smoking from one pregnancy to another although the reasons for this are not clear. It is important that we continue in our efforts to reduce cigarette smoking not only in women who are pregnant but also in our wider community."

Professor Philip Steer, BJOG editor-in-chief, said "Smoking is bad for your health, whether you are pregnant or not. It increases complications such as hypertension, heart disease and lung cancer.

"As a doctor, I would urge pregnant women to stop smoking as it harms the developing fetus. Every effort must be made to ensure that babies have the best start to their lives and this begins in the womb."


An abstract of the paper is available via this weblink.

BJOG: An International Journal of Obstetrics and Gynaecology is owned by the Royal College of Obstetricians and Gynaecologists (RCOG) but is editorially independent and published monthly by Wiley-Blackwell. The journal features original, peer-reviewed, high-quality medical research in all areas of obstetrics and gynaecology worldwide. Please quote 'BJOG' or 'BJOG: An International Journal of Obstetrics and Gynaecology' when referring to the journal.


Influence of previous pregnancy outcomes and continued smoking on subsequent pregnancy outcomes: an exploratory study in Australia.
Mohsin M, Jalaludin B.
BJOG 2008; DOI: 10.1111/j.1471-0528.2008.01864.x.

Royal College of Obstetricians and Gynaecologists