I remember reading something a while back on boa breeding seasons. If I remember correctly, it runs from about October-February, and once the female becomes gravid, it is about a 4-5 month process from then, which is why most boas give birth in the mid-late summer. If I can find where I read that, I'll post the link.
I found it. Here is the link where I read it...
http://superiormorphs.wordpress.com/...reeding-guide/
Hope this helps.