Pregnancy is not possible every day.
Each cycle, your body starts producing cervical fluid/mucus that helps sperm survive.
This fluid is a clue that your body's gearing up for ovulation (the release of an egg). For a number of days, you're potentially fertile.
After confirming ovulation by noting changes in fluid AND basal body temperature, you close your fertile window for the cycle and know it's not possible to get pregnant until next cycle.
In my FAM classes, we learn how to identify our own fertile windows!
We know when it's possible for us to get pregnant and when it's not, based on the data our individual bodies give us.
Then, we decide what to do with that info, depending on whether we want to avoid or achieve pregnancy.