Hi BARNEYKX, hope this explains a little
Shortly after their marriage, Catherine found herself pregnant. This first child was a stillborn daughter born prematurely in January 1510, but this disappointment was soon followed by another pregnancy. Prince Henry was born on January 1, 1511 and the was christened on the 5th. There were great celebrations for the birth of the young prince, but they were halted by the baby's death after 52 days of life. Catherine then had a miscarriage, followed by a short-lived son. On February 1516, she gave birth a daughter named Mary, and this child lived. There were probably two more pregnancies, the last recorded in 1518.
Henry was growing frustrated by his lack of a male heir, but he remained a devoted husband. He had at least two mistresses that we know of: Bessie Blount and Mary Boleyn. By 1526 though, he had begun to separate from Catherine because he had fallen in love with one of her ladies (and sister of one of his mistresses): Anne Boleyn.
It is here that the lives of Henry's first and second wives begin to interweave. By the time his interest in Anne became common knowledge, Catherine was 42 years old and was no longer able to conceive. Henry's main goal now was to get a male heir, which his wife was not able to provide. Somewhere along the way, Henry began to look at the texts of Leviticus which says that if a man takes his brother's wife, they shall be childless. As evidenced above, Catherine and Henry were far from childless, and still had one living child. But, that child was a girl, and didn't count in Henry's mind. The King began to petition the Pope for an annulment.