Menstrual cycle length and how it changes your due date

Cycle length is one of the few inputs that can meaningfully shift a due date, yet it's often overlooked. If your cycle isn't the textbook 28 days, the standard due-date formula can be off by several days — and it's easy to correct once you know your own number.

How to measure your cycle length

Your menstrual cycle length is the number of days from the first day of one period to the first day of the next. Count the first day of bleeding as day 1; the day before your next period begins is the last day of that cycle. Because cycles naturally vary a little month to month, an average over a few cycles is more useful than any single one.

What counts as a normal cycle length?

Cycle lengths vary widely between individuals. A commonly cited typical range is around 21 to 35 days in adults, with 28 days being an average rather than a rule. Regular cycles that sit anywhere in that range are generally considered normal; consistently very short, very long, or highly irregular cycles are worth raising with a clinician.

Why cycle length changes your due date

The standard Naegele calculation assumes a 28-day cycle with ovulation on day 14. If your cycle is longer, you ovulate later, so conception happens later — which pushes the due date back. If it's shorter, the due date moves forward. The adjustment is simply the difference from 28:

adjusted due date = LMP + 280 + (cycle length − 28)

So a 32-day cycle adds 4 days; a 26-day cycle subtracts 2. Our calculator applies this automatically — set your cycle length and the LMP-based dates shift accordingly. The reasoning behind the base formula is covered in Naegele's rule.

How long does the LH surge last?

Since people tracking cycles often use ovulation kits, a common question is how long the luteinising hormone (LH) surge lasts. The surge that triggers ovulation is typically brief — often detectable for roughly a day or so — which is why kits advise testing daily around mid-cycle so you don't miss it. Ovulation usually follows within a day of the surge. More on reading these signs is in ovulation timing and signs.

The takeaway

If you know your average cycle length, use it — it's the single easiest way to make a last-period due date more accurate for you. If your cycles are irregular, last-period dating is less reliable, and an early ultrasound becomes the better reference, as explained in how accurate is your due date.