From Mary Tudor, Renaissance Queen comes this analysis of Mary I's will written in late March 1558, when she thought she was pregnant:
During her reign, Mary perceived herself to be pregnant on two occasions. The final time was in 1557-8, the last years of Mary’s life and reign. Philip left England in July 1557, and by December Mary was confident enough of her pregnancy to write to him of the news. On this day [March 30th] in 1558, Mary made her will believing the birth was fast approaching (a due date of early/mid April appears to have been given). This was a customary procedure. Childbirth was rife with danger, so the prospect of the queen and her infant dying in the process was daunting though certainly not unthinkable.
Among her last requests was that she and her mother--QUEEN Kateryn--be reunited in their tombs at Westminster Abbey: ‘And further I will that the body of the vertuous Lady and my most dere and well-beloved mother of happy memory, Quene Kateryn, whych lyeth now buried at Peterborowh, shall within as short tyme as conveniently yt may after my burial, be removed, brought and layde nye the place of my sepulture, In wch place I will my Executors to cawse to be made honorable tombs or monuments for a decent memory of us’.
She also makes provision for the succession, should she die in childbirth: She leaves her realm to the ‘heyres, issewe and frewte of my bodye accordyng to the laws of this Realme’. So her successor is her supposed unborn child. Aware of the possibility of leaving the throne to an infant, Mary provides a regent. This was to be ‘my saide most Dere and well beloved Husband’. She lists her husband’s many virtues, especially his dedication to the Church. She asks for the loyalty shown unto her by her subjects to be transferred to her husband on the occasion of his regency.
Of course, now we know that she was not pregnant, that her half-sister Elizabeth would succeed her, and that she and Elizabeth would end up together in Westminster Abbey.