I am running through an alt history idea in my head, and I can't figure out where to find the exact thing. Hope you experts can help!
This might count as a Simple Question.
The Act of Settlement 1701, as amended, was and is the act of Parliament that controls the succession.
the Crown and Regall Government of the said Kingdoms of England France and Ireland and of the Dominions thereunto belonging with the Royall State and Dignity of the said Realms and all Honours Stiles Titles Regalities Prerogatives Powers Jurisdictions and Authorities to the same belonging and appertaining shall be remain and continue to the said most Excellent Princess Sophia and the Heirs of Her Body being Protestants
So, assuming no other changes, if Queen Victoria died while pregnant with no living children, and assuming that the child died, it would have been no later than 21 November 1840, the birth date of Victoria her first child.
Queen Victoria was the only (legitimate) child of the late Prince Edward, Duke of Kent and Strathearn, son of King George III, so that would have ended his line.
It would have gone to the next son of King George III. That would have been Ernest Augustus, Duke of Cumberland, King of Hanover, rejoining the personal union of the thrones of the United Kingdom and Hanover.
The laws of succession at the time passed the royal line through sons primarily, followed by daughters.
When George III died, the throne passed to his son, George IV. If George IV had surviving children, the throne would then pass to them. However, his only daughter died in childbirth.
As such, the throne would have passed to the next eldest brother, Frederick. Frederick had already died by this point and had no children to pass the throne to. So it went to the next brother, William IV.
William himself had no surviving children, so the next in line for the throne was the next brother, Prince Edward. Edward died in 1820, so couldn't take the throne but he did have a surviving child, Victoria.
Had Victoria died in childbirth with no surviving children, the throne would have gone to the next brother, Ernest Augustus, King of Hanover. Had he died , the throne would pass to his son, George V of Hanover.