Hi, I'm not sure. We can have an off month out of the blue where our cycle is different than the norm. I think too that once you are TTC, we get anxious and this compounds the difficulties in trying by delaying our cycle. My doctor told me to begin having sex on day 10 of my cycle and do so every other day for the next 10 days. That's the best way to make it happen regardless of a cycle that is messed up. If your cycle gets longer, just extend the number of days you do this. good luck