It exists in all fetuses but disappears after a point in development (or, as appears to be increasingly common, does not disappear). Rather than look for genet

It exists in all fetuses but disappears after a point in development (or, as appears to be increasingly common, does not disappear). Rather than look for genetic mutations as the cause (highly improbable given the time range) it is almost certainly an environmental trigger: mothers getting better nutrition during pregnancy.