It has this month been reported that the regressive form of autism does not tend to have an abrupt start (as commonly perceived, especially by those blaming vaccines for causing a sudden start of the condition). Instead the condition develops gradually from age 6 months up to 3 years (and possibly onwards as the study stopped there).
This is of course very compatible with the causation proposed in the update review of the antiinnatia theory, because that causation itself does not strike suddenly (like a vaccination shot) but instead builds up gradually with the infant's cumulating constant inhalation [breathing in] of the mother's dental mercury vapor.
The accompanying observation that parents tend to under-perceive their child's abnormality can be simply understood in terms of the universal natural reluctance to believe bad news. Which for a parent who already has an autistic child, is even badder news.