It's not a sure thing, but sounds like Guy B has better odds. I would trust an ultrasound over an app, especially if you haven't been keeping track on the app for a long time, and/or if your cycles are not the same length month to month. An app is just a collection of averages. Does it show your cycles come on the dot every 28th or 29th day, month in month out for years? If not, it shouldn't be used to contradict an ultrasound's measurements.