Toxins and poisons are considered to increase FA. Pregnancy sickness is argued to be an adaptation for avoiding toxins during foetal development. Research has reported that when a mother has no sickness or a sickness that extends beyond week 12 of gestation, the offspring shows higher FA (as demonstrated by measuring thigh circumferences). This suggests that when a mother fails to expel environmental toxins, this creates stress and developmental instability for the foetus, later leading to increased asymmetry in that individual. Greater exposure to pollution may also be a fundamental cause of FA. Research on skull characteristics of Baltic grey seals ("Halichoerus grypus") demonstrated that those born after 1960 (marking an increase in environmental pollution) had increased levels of asymmetry. Also, shrews ("Crocidura russula)" from more polluted areas show higher levels of asymmetry. Radioactive contamination may also increase FA levels, as mice ("Apodemus flavicollis") living closer to the failed Chernobyl reactor show greater asymmetry.