Would need to know how the sample was chosen for that test. If you sample women of reproductive age at random, there's decidedly not a 50/50 chance that she's

Would need to know how the sample was chosen for that test. If you sample women of reproductive age at random, there's decidedly not a 50/50 chance that she's actually pregnant: by rough estimation it should be about (1.75 [fertility rate] * 9 months [duration of pregnancy] / 30 years [reproductive lifespan] = 4-5%). If you give all of those woman a wheat/barley test and 70% of the positive tests were truly pregnant (implying about 6.25% test positives) that's actually pretty good, roughly 8x better than a coin flip. If you take a sample that's known to be half pregnant and half not pregnant and only 70% of the pregnant women are identified, it's decidedly less good.