From the perspective of someone who would have to run a study like this, a tremendous amount of pregnancies end before detection. To do actual pregnancy-initiat

From the perspective of someone who would have to run a study like this, a tremendous amount of pregnancies end before detection. To do actual pregnancy-initiation studies, you have to do absurdly intensive things like have women collect their urine on a frequent basis for analysis. Birth is a much less difficult outcome to collect for a large scale population estimate, as it usually just involves a data pull from vital records.