Tam- I didn't notice it during labour, either! It disappeared and came back again by Sept/Oct (born June), and that's when the physios took 3 months to give up on it. By the April of the following year I was seeing an Osteo, who was recommended through another osteo on the ABA forum.
I've also just had a timely email that might interest you SPD sufferers. Google oo.com and it's an Australian site selling overstocked items from major retailers. They have a thigh pillow, memory foam thing, selling for $12.95...I have one and it's BRILLIANT. You don't lose it through the night and it doesn't get in the way, because it just fits between the thighs
Osteo has given me stretches, but also strengthening exercises - you need both, not just stretches. A lot of the pain you start to develop comes from the muscles working hard to stabilise the joint, so if the muscles are conditioned properly, they'll support that instability so much better and will make a real difference to your quality of life.
Put it this way: I'm riding my horse again with no ill-effect, no 'sledgehammer up my crotch' feeling.