check how many married couples you still have. If you had a mass extinction, you might be running low on them and only married dwarfs produce offspring. I'm pretty sure the wiki has some excellent guides on how to force dorfs to marry.
Also, if you know the bug, you know it can be fixed. The vampire king is indirectly related. He's probably been feasting off the capital population for some time now, so your civilization is running low on able bodies. As a result, dorfs that have previously visited you as traders, guards or liaisons are appearing as migrants - which causes the bug.
Actually we had the king executed unintentionally during a mass culling of the goblin race...you see we had so many goblins caged up, that we needed to make a mass execution chamber...I must have accidentally assigned the King to this pit in my hasty pressing of the ENTER button. Anyway he's been dead for some time now...he was around during the year 1070 and died around....1074 i think? Anyway its 1081 now, late summer. I'll check the married couples though.
Also the other guy is right, i haven't given it much time since changing the baby ratio back to normal...I'll leave it running for a while and check back with you guys...
On another note, if every bug has a fix, where can i find a fix for the fucking trader bug? I know this belongs in the bug help board, but i've scoured that fucking place and have never seen a fix.