I've come very late to this thread and haven't managed to read everything on it, so apologies if these points have been raised already. It looks to me very much as though Alexandra's sister Irene and Heinrich of Prussia practised birth control after the birth of their haemophiliac first child Waldemar. They married on 24 May 1888 and Waldemar was born on 20 March 1889, so Irene was pregnant almost immediately, suggesting two highly fertile young people. Yet their second child, Sigismund, was not born until 27 November 1896, more than seven years later, and Heinrich, another haemophiliac, not until 9 January 1900. Unless there were several miscarriages in between, it sounds like birth control which was only partially successful.
Incidentally, by the time Heinrich married Irene, his elder brother the Kaiser had four healthy sons, and the Kaiserin was heavily pregnant with a fifth, so the chances of Heinrich's issue succeeding were low. If Heinrich was concerned that Irene was a carrier before the marriage, he was presumably prepared to take the risk, and the fact that his issue would proabably not succeed may have been a factor.
