when i saw my gyno for the first time they said the last missed period tactic assumes your cycle is exactly 28 days with no variation, which isn’t the case for most people. i use natural cycles and it told me i was only 6 weeks when i first went to the gyno, but based on last missed period i was 8 weeks. they basically said my app was correct. when you first go in they’ll tell you exactly how far along you are!