Q&A: Why is my son failing to thrive?

I'd first like to clarify a few terms so that what follows makes more sense.

Failure to thrive in infants and toddlers usually refers to the fact that they are not physically growing as briskly as we expect them to, or not at all. There is a very long list of possible reasons for this situation.

What you describe in his crawling and sitting later than most children we would refer to as developmental delay. The possible reasons here are also numerous, but not necessarily the same.

Developmental delays can result from reasons as diverse as not having a stimulating enough environment (maybe early on in your adopted son's life),to having an underlying disease, to having an abnormal brain. I presume it was this last possibility that someone was concerned about when an MRI was ordered, but I hope other tests were done as well.

As far as your question about whether not having a septum pellucidum is an explanation the answer is no. This finding, in the absence of other brain abnormalities, doesn't give you the diagnosis you need to either treat your son or to predict his future development. You need more information.

Consider taking him to a specialist in pediatric development to better uncover what may be going on.