Jane Seymour was Henry viii's third wife, and the mother of his only legitimate son, who became King Edward vi after his father's death in 1547.
Queen Jane started a very painful labor which lasted 3 days and 3 nights, not 9 days as in the song. She begged her nurses to "open my side and find my child" as in a caesarian section, but Henry would not give permission, as it was a dangerous procedure for mother and child in those days. There was a rumor that he eventually allowed the C-section because she was in extreme pain. Edward was born on October 12th 1537 to much rejoicing. But 9 days later his mother Queen Jane died of puerperal fever, which was quite common and probably due to an infection during her long labor.
Because she gave him a son, and caused him no trouble, Henry viii called her "my most beloved Queen" and was buried beside her.