If you always change the ring on time, it can be very effective. With perfect use, it’s more than 99% effective. The way it’s typically used, though, it’s around 91% effective.

In other words:

Of those women who use the ring exactly as directed, fewer than 1 in 100 will experience an accidental pregnancy during the first year of using this method.

Of those women who do not use the ring exactly as directed, 9 in 100 will experience an accidental pregnancy during the first year of using this method.