Taking the questions in order:
1. No, in general, a disorientate head to body ratio doesn’t cause muscular or developmental problems in an otherwise healthy child. The size of our heads is strongly determined by our genes. Big heads run in families, and, alone, don’t interfere with development.
2. Does the difference in in vitro movement mean anything? Yes and no. The problem is that fetal movements are very non-specific, affected by the baby’s well-being, but also by temperament, positioning, and the experience of the mother. Too many variables mean this sign alone can’t be relied on too much.
3. This question seems to be the key one. I asked a neurologist at a large pediatric hospital about the association between a large head and PDD. Apparently, there is a known correlation, at least after birth. But given how strongly correlated family head size is (you mentioned large heads run in your wife’s family) it wouldn’t be fair to make too much of this correlation.
We have a lot to learn about the PDD/autism spectrum. Depending upon the definition used, between 4 and 21 children out of every 100,000 carry this diagnosis. Boys outnumber girls by 3 to 1, and even more so when the symptoms don’t include mental retardation.
We now believe that there must be some genetic component, because having one affected child means there is a 3 percent chance, overall, of having another affected one. (But there is a 97 percent chance of not having one, too.) There are certain inherited syndromes that involve PDD symptoms, though not necessarily a large head to body ratio. At some point, you may want to ask your primary care doctor for a referral to a geneticist or a pediatric neurologist.
Your questions are good and important ones, and those two specialists can best provide you the most updated answers.