only accurate way to tell is a progesterone test
mother nature is pretty accurate herself. life has been spreading way before chemical testing.
i agree about mother nature but not every female is the same. My last dogo breeding her levels werent ready until day 17. Some are ready early and some late. The progesterone test isnt the only way but def the most accurate. In almost all cases tho if you let them tie up every day or every other day after the first week she will be prego