The first estrus cycle usually occurs by age 6 to 12 months; for some cats as early as 4 months of age, for others not until age 12 months or so. (thesprucepets.com)
After many years of estrus cycles without pregnancy, the uterine wall undergoes the changes that promote this disease. (michigananimalhospital.com)
There may be additional charges for non-routine surgery, which would include a pregnant or in estrus cycle (heat), infections, undescended testicles, false pregnancy, obesity or females that have had previous litters. (oc-paw.com)