Many studies have attempted to assess the effects of smoking during pregnancy on maternal and child complications. Adverse consequences, including premature birth, low birth weight and long-term sequelae including developmental problems such as cognitive delays have been connected to maternal smoking [1, 2].
A variety of factors have contributed to making it difficult to evaluate the effects of smoking during pregnancy. These include errors in measurement such as maternal denial and under-reporting, fluctuating behaviour of smoking during pregnancy and metabolism, including accelerated metabolism during pregnancy .
It is important clinically to have knowledge of patients smoking habits since it enables appropriate anti-smoking advice to be given and pregnancy is seen as a window of opportunity to provide this. Meta-analyses, including a Cochrane review, show that appropriate screening plus active intervention, in the form of advice and provision of written materials, increases the numbers of pregnant women who stop smoking. This in turn can reverse the adverse effects of smoking on perinatal outcomes by up to 20% [4, 5]. It has also been emphasised that, in pregnancy, biochemical validation can increase women's motivation to stop smoking and increase their utilization of available treatment services , though a recent review showed no evidence that biofeedback motivated cessation .
In the general population, the proportion of people who report to be non-smokers but show biochemical levels indicative of smoking are generally low . In pregnancy, however, women who smoke can experience intense social and clinical pressure that results in false declarations of non-smoking. This inaccuracy of self-reported smoking makes appropriate counselling difficult and stresses the need for reliable biochemical confirmation of smoking status .
Of the biochemical measures to assess smoking, cotinine, a metabolite of nicotine found in the blood and urine, is most preferred by scientists because of its long half-life, averaging 17 hr in non-pregnant women  and 9 hr in pregnant women . In the clinical setting however, breath carbon monoxide (CO) level is seen to provide an immediate, non-invasive method of assessing smoking status and is the method most suitable to the antenatal clinic . It is less preferred as a biochemical marker because CO has a short half-life of approximately 1–4 hours  and may not detect low levels of smoking . Furthermore, the development of relatively inexpensive portable CO monitors enables breath CO levels to be assessed in a wide variety of clinical settings .
Currently in Glasgow, all women attending the antenatal clinic are CO monitored and those with a reading of ≥ 8 ppm or self-reported smokers are directly referred to a smoking cessation link midwife for a 6 week support programme. Since the results of the CO test and or self-report are the critical factors for referral for cessation advice, the accuracy of these is paramount to providing intervention. The 8 ppm cut-off in this programme is based on a standard cut-off for abstinence verification that is widely accepted within the research community . New evidence suggests that this may not be appropriate in all settings. Recently, Javors et al concluded that the cut-off should be lowered to 2 or 3 ppm for assessment of smoking in a general population and a cut-off of 4 ppm in post partum women was suggested [17, 18].
The present study examines the sensitivity and specificity of the breath CO test, and the optimal cut-off level to distinguish smokers from non-smokers amongst pregnant women.