How effective are internal condoms at preventing pregnancy?

That all depends how they’re used. With perfect use they are around 95% effective, but the way they’re typically used you can expect around them to be closer to 79% effective.

In other words:

Of those women who use the internal condom exactly as directed, about 5 in 100 will experience an accidental pregnancy during the first year they use this method.

Of those women who do not use the internal condom exactly as directed, about 21 in 100 will experience an accidental pregnancy during the first year they use this method.