I understand what your coach is saying -- there are hormones that start loosening your joints so that the pelvis is flexible enough for the baby to pass through. I didn't NOTICE this happening so much with my first child, but I did with the second -- and I can see the possibilities. I didn't ride back then, but I sure felt "wobbly" in my hips and pelvis when I'd go walking that second time around!
I don't if one can call chronic pain a given, but I DO still have some issues that flare up on occasion 23 years later, and I didn't "abuse" (for lack of a better word at the moment) my body like I could (and probably should) have back then!
I think that if you talk with your OB or midwife as Mary suggests, he or she will do some research for you if needed, in order to give you the most appropriate answer. You might also ask your coach for some literature to read and understand better what he says is going on with your body. He may also be able to suggest a new riding style -- sounds like you ride pretty aggressively, and maybe a more relaxed approach, or different seat style for the duration, could keep you riding? (maybe recumbent?)
Even if you do stop riding for now, it's a small price to pay for the long term benefits!
Have you had that 20 week ultra sound yet? Boy or girl? Adorable? Our grandkids' pictures at that age were pretty cute, especially the ones of the youngest, little Orion -- he poses so well for the cameras! (This is our wee biker's little brother, for those of you who remember Xander in his flaming helmet!) I for one would LOVE to see your baby pictures if you've got them!
Karen in Boise