Does CD 16 O=CD 33 AF?

[Springermommy]

Hello, my cycles vary from 31-39 days. I took an OPK this month and got a relatively dark line on CD 15 and a very dark line on CD 16. Does this mean I should expect AF on CD 32?
FF says she won't come til CD 38.... This is so confusing!

 

[Dill]

There's no fast-and-easy answer to this question based on average cycle length alone, I'm afraid! It depends on what your typical luteal phase (the time between ovulation and menstruation) length is, which can vary widely from woman to woman, but generally remains consistent to the individual (so no matter how long your cycle length is between one month and the next, you'll have the same luteal phase length). If you know your average luteal phase length, then it's easier to figure out when AF is due.