Well, most importantly, if you stop taking the birth control pill, you could become pregnant, even if you only miss one.
If you stop because of side effects, inconvenience, cost, or because you hope to get pregnant, you may find it takes a few months for your menstrual cycle to return to how it was before you started taking the pill. Sometimes it takes a few months to ovulate, sometimes you ovulate very quickly after stopping. The hormones from the pill are out of your system in a few days, regardless of how long you’ve taken it.
Some of the side effects of the pill may go away, so you may notice some changes in your body. For example, often the pill can improve skin by reducing acne, so you may find that acne returns. If you gained a few pounds by going on the pill, you may lose them when you stop taking it.