Because such extraordinarily complex structures develop out of very simple cell groupings, early problems in neuro-development can lead to severe birth defects. For example, spina bifida is linked to defects in the neural tube. In fact, serious problems in the first trimester of pregnancy often lead to miscarriage, such that 80 percent of miscarriages occur in the first trimester. When we consider the enormous lengths the brain must travel in its journey to maturity, it is indeed remarkable that so many human beings are born without brain damage.