Does my First Morning Urine affect the results of ovulation test?

They do suggest you don't use FMU and I think it's because it can be 'too concentrated'. The best advice I have heard is to not urinate or drink any water from 1 PM - 3 PM and test at 3 PM. I suppose it is possible you have a false positive, but it could also be accurate. An 18-20 day cycle is very short. You're counting from first day to first day, right? The day it stops doesn't come into play.