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How Nadi works

How we calculate your cycle

Nadi works from two things you tell us: how long your cycle usually is, and how long your period usually lasts. From those, we map the four phases of your cycle so you can plan around them. Nothing is hidden, and nothing is guessed from data we have not asked you for. Here is exactly how we do it, and just as importantly, what it can and cannot tell you.

Day 1Day 31

An example cycle of 31 days with a 5 day period. Your own map is built from your own numbers.

The four phases

  • Period
    Day 1 is the first day of your period. It lasts as long as your period usually does.
  • Follicular
    The stretch after your period while your body gets ready to release an egg. This is the part of the cycle that naturally varies most, so it is the part we let stretch or shrink.
  • Ovulatory
    The few days around when an egg is released. This is the most fertile part of the cycle.
  • Luteal
    The roughly two weeks between releasing an egg and your next period. This part tends to stay steadier from one cycle to the next.

Why we count back from your next period

You might expect us to count forwards from day 1 and drop ovulation in the middle. We do it the other way around, and there is a good reason.

Research has found that the second half of the cycle, the luteal phase, tends to stay fairly steady, usually around two weeks. The part that stretches and shrinks is the first half, before an egg is released. So when your cycle runs longer or shorter one month, it is mostly the early part that changes, not the end.

That is why we anchor the last part of your cycle and let the earlier part flex. It matches how cycles actually behave, rather than forcing everything around a fixed midpoint.

Why we do not just say "day 14"

You may have heard that ovulation happens on day 14. For most women, it does not.

In one large study, only about one in eight women actually ovulated on day 14. Ovulation can happen earlier or later, and it can shift from one month to the next with things like stress, travel, illness, or simply how your body is that cycle. Another well known study found that the textbook fertile days only matched reality for around a third of women.

So instead of pretending we know the exact day, we map a short window where ovulation is most likely for someone with your cycle length. It is an honest best estimate, not a precise prediction.

When your cycle is short

If your cycle is on the shorter side and your period is on the longer side, the phases can start to bump into each other. There may be no quiet gap between your period ending and your fertile window opening, so the follicular phase can show as zero days. This is normal, not a mistake.

In a very short cycle, the fertile window can even begin while your period is still finishing. When two phases land on the same day, we show the period, because that is the part you can actually feel and see. Either way, your map still adds up to your whole cycle, with every day belonging to just one phase.

The honest pros and cons

What it is good for

  • Giving you a clear, simple picture of your cycle from things you already know.
  • Helping you plan ahead, notice patterns, and understand your body a little better.
  • Staying fully transparent, with nothing measured or assumed behind your back.

What it cannot do

  • It cannot pinpoint the exact day you ovulate. That needs signals like body temperature or hormone tests, which we do not ask you for.
  • It cannot know that a cycle has turned out longer or shorter than usual until you tell us.
  • It should not be used to avoid or plan a pregnancy. For that, please speak to a healthcare professional or use a method made for it.

What to keep in mind

Your body is not a clock. The phases we show are a helpful guide based on your usual pattern, not a fixed schedule. Some months will look different, and that is completely normal. If anything about your cycle worries you or changes a lot, it is always worth talking to a doctor.

How we keep it honest

  • We only use what you tell us. Your cycle map comes from your own numbers, nothing else.
  • We map three cycles ahead, then check in. The further ahead we map, the less reliable it becomes, so we keep it short and ask you to update your details every few cycles.
  • We keep a history of your changes. Each time you update your cycle details, we save it so you can look back and see how your own patterns shift over time.
  • We tell you when something is an estimate. We would rather be clear about what we do not know than sound more certain than we really are.

If you have any questions

You do not need to work everything out on your own. There is more about how Nadi maps your cycle across the site, and if something still does not make sense, or your map ever looks off, you can write to us any time at hello@nadi.health. We would always rather hear from you than leave you wondering.

The research behind this