Observational data suggest that 20–40% of female smokers quit during pregnancy (2). Smoking cessation during pregnancy is influenced by multiple factors, including maternal age, socioeconomic position, parity and partner's smoking (5,6). In addition, genetic susceptibility to the addictive properties of nicotine is likely to be important. Evidence from twin studies suggests that up to 70% of the variance in nicotine dependence is due to genetic factors (7). Consistent with this, a study of 840 adoptive families demonstrated high concordance between the smoking behaviour of adoptees and their biological full siblings (8).
