Seems like it a more common occurance than what people think. I was surprised at how many women I know (family members,friends, other women) have had miscarriages. I can think of almost 10 women just off the top of my head that I know who have gone through this.
I would imagine if the average person knows several women who have had a miscarriage, no doubt most royals probably know of a family member or another royal whose had one as well. I would be very surprised if any of them ever discussed it at great length among themselves.
I remember being at a job where this woman talked at length about her 2 miscarriages going into great detail about the whole thing. I remember being very uncomfortable listening to her talking about it at length as did several other women who were in the room. No one knew what to say to her which is why this topic of conversation is avoided.