false-positive result. This is because while pregnant smokers may falsely report that they have quit, the reverse situation, in which pregnant women falsely report smoking, is extremely unlikely. Women who continue to smoke in pregnancy are expected to have a higher frequency of the risk allele due to higher nicotine dependence. If a proportion of these women falsely declared that they had quit smoking, this would cause the group of ‘quitters’ to be enriched with the risk allele, causing the association to be biased towards the null. Another limitation is that our study participants may not fully represent the general population. We have previously shown that Exeter Family Study of Childhood Health (EFSOCH) study participants had a lower-than-average level of socioeconomic deprivation and were more likely to be non-smokers (20). However, the rs1051730 SNP is not associated with these factors, so this will not introduce bias.
