Extreme prenatal maternal stress is a biologically plausible risk factor but only a few studies have examined this effect. For example, a Danish study reported an association between adverse prenatal maternal life events (eg. severe emotional stress such as bereavement) and cranial-neural-crest malformations. In a Finish study, offspring of mothers whose husbands died during the pregnancy (mostly during wartime) had a significantly increased risk for schizophrenia and other psychiatric disorders. Also the 1940 invasion of the Netherlands was associated with an increased risk for schizophrenia among those in utero at that time. One explanation for these findings is excessive release of maternal cortisol, which may be toxic to the developing fetal hippocampus.