In 1527 Queen Bona Sforza of Poland suffered a miscarriage when she fell from her horse. The cause of her fall was that a bear her husband had been hunting, charged at the royal couple. The labor was a child with the name of Wocjiech (whom some sources refer to as Olbracht, Adalbertus or Albert) who promptly died shortly after birth.
As we all know, Bona's only son, Sigismund II was the last of the Jagiellonian kings of Poland. But what if she had not been with on the hunt (since the birth rendered her barren)? No falling from her horse means no miscarriage, and a little bit of luck she can birth the child at full term.
What does the future of the Polish monarchy look like if Sigismund is not destined to be the last of the Jagiellons?