Don't count your chickens before they're hatched.

proverb Don't make plans based on future events, outcomes, or successes that might not come to pass. When my mom heard that I was preparing my campaign before even being nominated, she warned me, "Don't count your chickens before they're hatched." Why are you begging to drive my car to school tomorrow when you still need to take your license test in the morning? Don't count your chickens before they're hatched, babe! Don't count your chickens before they're hatched, man. Harvard's a really hard school to get into.