While male iguanas will begin to slack off a bit in the fall, females who come into season in the fall will start to pack away the food in preparation for egg production. The eggs get so big and take up so much room in their bodies that by the second month of gestation, their food intake has decreased to almost nothing beyond a few sips of water and some leafy greens or a mouthful of salad. It is important to boost calcium intake during this time. You can do this by adding slightly more calcium supplement to her food, or giving her the supplement more often. One of the most common problems with females in captivity is egg binding resulting in C-section and hysterectomy.
Many vets actually recommend spaying females routinely to prevent the problem.
It is, of course, less costly for you and less stressful for the ig if she can just get the exercise she needs. She will be getting fatter with the eggs as she is losing weight, so she won't look much different, albeit her belly and sides will be more taut. You will be able to feel some of the eggs along her sides before they are developed enough for you to be able to see them. Generally, a row of two eggs on each side will be felt. They then back out and back fill the cavern and burrow.