Infertility as a phenomenon is sometimes curable and sometimes incurable, leading to negative emotional impact on couples Delpisheh et al., 2007Direkvand-Moghadam et al., 2015Direkvand-Moghadam et al., 2016Sezgin et al., 2016. To date, about 10-15% of all couples suffer from infertility Sarvari et al., 2010. An increased prevalence of infertility around the world has caused worry in the general population. Efforts have been made to evaluate the factors affecting pregnancy outcomes Sépaniak et al., 2006. Tobacco use is considered a high-risk behavior. Indeed, 30% of females and 35% of males of reproductive age in the United States are addicted to tobacco smoking The Practice Committee of the American Society for Reproductive Medicine, 2008. In fact, primary infertility and history of primary infertility have been reported in 3.6% and 1.7% of tobacco smokers, respectively (Delpishe et al., 2014).

A single cigarette contains 400 chemical compounds, some of which are toxic and carcinogenic. Nicotine, carbon monoxide, tar, arsenic, ammonia and hydrogen cyanide, are the ingredients in cigarettes, which additional can contribute to heart and lung injury. These substances have a great impact on the human reproductive system Cooper and Moley, 2008. So far, several studies regarding the impact of tobacco on human fertility have been carried out; however, conflicting results have been reported.

Some of these studies have reported a negative impact of smoking on male and female fertility Dechanet et al., 2011Oyeyipo et al., 2011. Indeed, smokers are more likely to have fertility problems than non-smokers. If a user smokes for many years or smokes many cigarettes per day, the risk for fertility problems increases (Amirkhani et al., 2014, Kim et al., 2015). However, other studies have reported that there is no significant relationship between smoking and fertility outcomes in humans de Jong et al., 2014.

Smoking causes damage to DNA and leads to reduced sperm quality and induction of male infertility Ernst et al., 2012. Passive smoking also causes a decrease in sperm motility and interference in sperm acrosome function Arabi and Moshtaghi, 2005. One study reported that tobacco exposure during pregnancy can cause long-lasting effects in the reproductive system Håkonsen et al., 2014. One such effect is the increase in ovarian dysgenesis in women who are exposed to tobacco during their fetal period Uzumcu et al., 2012. Additionally, chemicals in cigarette smoke may accelerate follicular depletion in smokers and fetal exposure to cigarettes (which can reduce testosterone levels in girls) Delpisheh et al., 2008Strohsnitter et al., 2008.

The impact on reducing the age of menopause onset may be due to the decrease of vitamin and mineral absorption Hatch et al., 2011. According to one study, tobacco use likely increases infertility by 1.6-fold Hatch et al., 2006. Berthiller and colleagues, however, reported that fertility did not decline in both male and female passive smokers Louis et al., 2011Oyeyipo et al., 2013.

In light of the inconsistent results of some studies in relation to the impact of tobacco use on fertility, the aim of this study was to investigate the relationship between cigarette smoking behavior and infertility in women. The Fagerstrom Test for Nicotine Dependence (FTND) was used to evaluate multifactorial factors comprising two or more distinct factors.

In this case-control study, 350 female subjects (including 173 fertile and 177 infertile women) were enrolled. The mean age of fertile and infertile women were 20.79 ± 3.51 and 19.65 ± 2.79 years, respectively, with no significant difference (P = 0.157). Of the subjects, 70 (20%) participants had the experience of smoking at least once in their life. The experience of tobacco use in infertile women was greater than in fertile women (23.7% versus 16.1%, respectively; P = 0.012). The experience or attitude towards smoking for the fertile and infertile groups are presented in Table 1 .

In Table 2 , the smoking habits of the fertile and infertile groups were broken down into the following components: number of cigarettes smoked daily, age at first smoking experience, the best time to smoke during the day. Indeed, 26.1% of infertile women smoked 6-30 cigarettes daily; this number was significantly higher than for fertile women (p= < 0.001). This difference was also observed with 1-5 and 31-60 daily cigarettes but to a lower degree.

Among the smokers, 66.7% of infertile and 76.9% % of fertile women obtained the first experience of smoking at the age of <10 years old but this difference was not significant (p=0.674). When we compared infertile and fertile groups with regards to the best time to smoke during the day, the results indicated that infertile women smoked in mornings more than fertile women (p=0.024).

The results of the Fagerstrom Test are shown in Table 3 . “Low Nicotine” dependency was found to be more frequent in all study subjects than was “Moderate Nicotine” or “High Nicotine” dependency. Moreover, low nicotine dependency was significantly greater in infertile women (p=0.012).

Thus, the significant factors that predicted infertility are: smoking in the morning hours of the day, low dependency compared to moderate and high dependency, and positive history or experience of smoking ( Table 4 ). A positive history of smoking increased the odds of being infertile (OR 2.88; 95% CI: 1.56- 4.92). Regarding the time of the first cigarette smoked during morning hours, it was found that for women who smoke their first cigarette immediately after morning wake-up, their infertility risk was 5.14 times more than that of other women (95% CI: 1.17-22.48). The risk for infertility is associated with low nicotine dependency (OR 3.12; 95% CI: 1.16-8.09).

This cross-sectional study aimed to examine the relationship between cigarette smoking behavior and infertility in women. Based on the results, 16.7% of tobacco consumers have high nicotine dependency. Moderate and low nicotine dependency was reported in 14.7% and 82.7% of tobacco consumers, respectively. The results showed that 20% of the participants in the study have mentioned a history of tobacco cigarette experience at least once. Despite the various harms that smoking causes to the health of the community, one-third of the world's population over 15 years old permanently smoke cigarettes Sépaniak et al., 2006Strohsnitter et al., 2008. The results of our current study also showed that the rate of nicotine dependency in infertile women was greater than that for fertile women.

The intake of nicotine has a dose-related effect which lead to delays in fertilization via effects on hormone production in females and early menopause (which reduces the length of fertility). The possibility of spontaneous abortion with pathologic mechanisms as a result of cigarette smoking is another dangerous outcome in female smokers Weigert et al., 1999. Studies have shown that high levels of nicotine can induce cellular apoptosis of ovarian follicles, resulting in increased infertility in women consuming tobacco Harlev et al., 2015Uzumcu et al., 2012. On the other hand, other ingredients in cigarettes can affect the anatomy and function of the uterine tubes, causing increased infertility in tobacco consumers Oyeyipo et al., 2014. However, in contrast to our findings, one study reported no significant relationship between smoking and fertility Oyeyipo et al., 2013.

Based on the results of our study, most infertile women declared that the best consumed cigarettes occurred in the morning period. Our analysis reveals that infertile women have a great dependency on nicotine. Moreover, based on our results, 16% of infertile women have mentioned extreme dependency to nicotine, while none of the fertile women reported extreme dependency to nicotine. A laboratory study analyzed the impact of the different concentrations of nicotine on fertility in rats and showed that nicotine had a dose-dependent effect on fertility power Louis et al., 2011.

There were some limitations in our study. Firstly, interpretations from this study should be considered with caution since the data was derived from self-reporting items on the questionnaires. Secondly, recall bias is a factor for most case-control studies; it was for our study as well. Finally, our study was limited to only those subjects who passed the selection criteria.

In summary, nicotine dependency increases the risk of infertility. In light of the effects of infertility on the physical, mental, social and the economic aspects in the community, training on the negative effects of tobacco consumption on fertility should be included in the educational packages designed for health care providers.

DNA: Deoxyribonucleic Acid

FTND: Fagerstrom Test for Nicotine Dependence

OR: Odds ratio

This project was financially supported by Psychosocial Injuries Research Center, Ilam University of Medical Sciences.

Study concept and design: MS, YV. Analysis and interpretation of data: AM, AD, AD-M. The drafting of the manuscript: YV, AD-M. Critical revision of the manuscript for important intellectual content: YV, KS, MA. Statistical analysis: YV, KS. Study supervision: KS. All authors approved the final manuscript.