Disadvantages:
It’s unreliable. You can try to pinpoint the day of ovulation, but it may occur during every day, due to variations, like medication, stress, crash diets etc. With perfect use about 9% of the girls becomes pregnant every year. But in reality it is at least double this figure.
Every month you have to abstain from intercourse for many days. You must be very motivated and have good self control to keep to the rules.
If a girl’s cycle is very irregular there will be many days of abstinence, making this method of minimal use.
You have to keep track of the day your period starts and how many days ago this happened. This takes a lot of motivation and leaves little room for spontaneity.
Doesn’t protect you against STD’s.

Advantages:
You don’t need to go to the doctor.
There are no side effects.