Pregnancy Tests

There’s an advert for a pregnancy test doing the rounds at the moment, and the small print at the bottom of the screen says the test is 70% accurate. Aside from the 3 in 10 chance of it being wrong apparently making it a pointless exercise, I wonder what percentage of people who take a pregnancy test are actually pregnant. Somebody must suspect they are if they’ve even bought the kit, so you’d expect the majority to get a positive result. Could you sell a test that always gave a positive result, and still claim a 70% success rate?