I really dont know exactlly when does it start, but Mocha was schedule to get spayed and (at 7 months) and she got her first heat cycleShe also got so very sick(almost died on me) because the amount of fluid retention in her uterus. The vet thought for sure it was pyometra.she was at the Vets for 2 days hooked to IVs,they did blood chemistry diagnostics,cbc,pancreatitis tests and so on. I could have prevented all this had I scheduled her surgery sooner. I highly recomend that you get her spayed as soon as posible. it is not worth the risk of something happenning to your baby.It costed us so much money when this happend to Mocha and I could have prevented it with only 250.