when a dog goes on heat the second time?

do their vulva always swell to the same size they did the first time?my dogs vulva is swollen but for the past days it hasnt grown any bigger. last time it was huge!!she also has all the other symptoms of going on heat and is due.we arent breeding her. we were hoping to get her spayed but cant now coz she is going on heat.do their vulvas swell to the same size everyytime?

Dogs usually go into heat ever six to nine months.

No; not every time. It's perfectly normal to be smaller than the first time, or bigger than the last time.