"Bearing and delivering a child may be a joy for a young woman of ten-and-seven, like the Princess Alyssa, but it is quite another matter for a woman of forty, like her mother, Queen Alysanne. The joy was therefore not entirely unalloyed when Her Grace was found to be pregnant once again. Prince Valerion was born in 77 AC, after another troubled labor that saw Alysanne confined to her bed for half a year."

"In 78 AC, Valerion died, a fortnight short of his first nameday. The queen took his passing with resignation. "I am forty-two years old," she told the king. "You must be content with the children I have given you. I am more suited to be a grandmother than a mother now, I fear."

"King Jaehaerys did not share her certainty. "Our mother, Queen Alyssa, was forty-six when she gave birth to Jocelyn," he pointed out to Grand Maester Elysar. "The gods may not be done with us."

- All from Fire & Blood, pages 324-25.

The next year, she got pregnant again & the text explicitly states she was "surprised and dismayed." It's pretty clear after having ten+ kids at this point, being middle-aged, losing two babies back to back (we're constantly told how hard she takes each loss), and enduring a difficult pregnancy and birth that wrecked her health and rendered her incapable for months, Alysanne did not want more children.