Why does my Fertile Window move?

Gunce • Head of research at Glow. Unwilling infertility expert. 2 kids after 6 IVF treatments.

You might have noticed that your fertile window in Glow is not set in stone. It can move away, before you ever get to it, so you feel like you are playing catch up. And it can shift even after it has passed, which can be frustrating because now you might have had intercourse at the wrong time. So why does it move?

Well, frankly, as frustrating at it can seem at first, the fact that your fertile window moves is what makes Glow such an excellent and successful fertility app. Glow learns from you. Glow analyzes your data. Glow self-corrects and comes up with better predictions for the future. 

But let's look at it a bit more closely. 

Let's say you have the typical 28 day cycle and are expecting your ovulation around CD14. Well, in the absence of all other information, so will Glow.  But then let's say you start entering some data - maybe your CM, or BBT, or ovulation tests. Or even other symptoms. Glow will examine this data and actually move around your expected ovulation based on it. It might move it closer to today. Or further away. For example, if you are CD12 and you keep telling Glow that you have very little CM, or negative ovulation tests, believe me....Glow will use that information and push out your fertile window. 

Now let's say that you and Glow decide finally that you ovulated on CD15. Great. For the interim, your fertile window locks into place. That is until if and when you miss your period. At that point, several things come back in to question. Such as, why is your period late?

1. You could be pregnant. (But if that is the case, please tell Glow.)

2. Glow might have miscalculated your correct day of ovulation. Glow assumes this because the one consistent part of every woman's cycle is the Luteal Phase. (The period of time that lapses between ovulation and your next period). For almost all women this is 14 days, with a standard deviation of  one or two days. So if your miss your period, especially by 5 or more days, the correct assumption is that you did not ovulate when you think you did. 

So if you miss your period, Glow will start dragging out your past fertile period in lockstep. It will simply assume that the data that led to that original conclusion was not ideal. This can be very frustrating for you - but please believe that it is actually in your benefit. Because now Glow uses this new information to more accurately forecast your next cycle and you have a better chance of getting pregnant than if it had maintained that your fertile period happened when you initially thought it did. 

We hope that makes sense. Please let us know if you have more questions on this subject. 

Thank you!