Exposure to second hand smoke (SHS), active or passive, is considered the single most important cause of avoidable morbidity and early mortality in many industrialized countries . Over the last years, Spain presented a decreasing trend in smoking not only in the general population but also during pregnancy [2–5]; however it is still an important risk factor for infant health. Most recent population data from Barcelona documented that 28% of pregnant women smoke at the beginning of pregnancy, and although 42% of them quit during the gestational period, 16% smoke throughout pregnancy  therefore the gestational and fetal effects of cigarette smoke are of great importance for public health . The effects of tobacco smoke exposure on the health of pregnant women are associated with an increased risk of spontaneous abortion, low birth weight, prematurity, perinatal death and sudden infant death syndrome [7–9]. while the risk of cognitive problems and neurodevelopment growth , childhood cancer  and respiratory and allergic symptomatology in the first years of life  can be increased as well.
There is scientific evidence that smoke-free environment is the only strategy protecting the population from the negative effects of SHS . For this reason, several countries have implemented legislations requiring all enclosed workplace and public places to be free of SHS . Ireland was the first country with comprehensive smoke-free legislation implemented in 2004. Since then, countries like Norway, New Zealand, Italy, Uruguay, England and many provinces or states in Canada, the USA or Australia [15, 16] followed. Recently, by measuring air nicotine concentrations, a study had shown that exposure to second hand smoke has decreased greatly in indoor public places and workplaces in Uruguay after the implementation of a comprehensive national smoke-free legislation .
Spain introduced the legislation to reduce SHS in 2005. The Law 28/2005 includes a health recommendations against smoking and recommendations forregulation of tobacco smoking in public places with wide exemptions in bars, restaurants and night clubs [18, 19]. Furthermore, the implementation of smoke-free legislation generated a continuous discussion topic in the media . After the implementation of the Spanish smoke-free legislation [3, 4, 21–23], programs to help smoking pregnant women to quit were implemented (program "Embaràs fum") in Catalonia. These programs included specific training and free nicotine replacement treatment for pregnant smoking women throughout pregnancy .
Prenatal tobacco exposure has been usually assessed using self-reported maternal questionnaire [25–28]. Difficulties in differentiating passive tobacco exposure from active smoking (such as reluctance to admit active smoking or being unconscious of passive exposure) have prompted the use of specific biomarkers to prevent bias in self-reporting questionnaires [29, 30]. Nicotine and its main metabolite (cotinine) have been used as biomarkers for SHS in conventional (blood and urine) [31, 32] and non-conventional matrices (saliva, meconium and hair) [33, 34]. Cotinine presents a longer biological half-life than nicotine and it is considered the best biochemical marker to differentiate between active and passive tobacco exposure [33, 35–37]. Moreover, the levels of cotinine have been found to be directly related to daily cigarette consumption . Cotinine cord blood is the most sensitive and least invasive measure of prenatal SHS in newborns [36–42].
The present study examines the possible effects of the actions taken to prevent prenatal SHS exposure. The relationship between the implementation of the law against smoking and a decrease in maternal tobacco use and exposure during pregnancy has been studied by measuring cotinine in cord blood as a reliable biomarker of active and/or passive maternal exposure to SHS. In addition, the relationship between the newborn's cotinine level in cord blood and the parents' smoking pattern declared by questionnaire has been also evaluated.