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MasterMath

Late Period Calculator

If your last period started on 1 August and your cycle is 28 days, by 9 September you are eleven days late and, if pregnant, you would be five weeks and four days along. Work out the exact delay and what it means.

You are late by

—

You are late by—
The period was due on—
If pregnant, you would be—
A test would be reliable from—
If not, the next one is due—

How this was worked out

    Indicative result. A late period has many causes besides pregnancy: stress, weight change, hard exercise, medication or thyroid problems. If it keeps happening or comes with other symptoms, get it checked.

    The formula

    days late = today − (last period + cycle length)

    Where it comes from

    The delay is counted from the day the period was due, which is the first day of the last one plus the length of your cycle, not from the period itself. The difference matters because 28 days is not everyone's cycle: on a 32-day cycle, what looks like four days late is actually zero.

    How to work it out by hand

    1. Take the first day of your last period
    2. Add the length of your cycle: that is when the next one was due
    3. Subtract that date from today: that is the delay
    4. For weeks of pregnancy, count the days since the last period and divide by seven

    What is worth knowing

    Two or three days is not a delay: a cycle shifts on its own for reasons as ordinary as a trip or a bad week of sleep. And one thing catches most people out the first time: weeks of pregnancy are counted from the last period, not from conception, so by the time a test turns positive you are already four or five weeks along. Those first two weeks come before ovulation, and they count anyway because the last period is the one date almost everyone knows for certain.

    Frequently asked questions

    How many days late is a concern?

    Up to three days is within the normal variation of a cycle. After a week it is worth taking a test, and after three months without a period, worth getting checked.

    Can I be pregnant without being late?

    It is uncommon, but implantation bleeding can be mistaken for a light period. If in doubt, a test settles it.

    Why are weeks counted from the period?

    Because it is the date known with certainty. Conception is estimated; the last period is remembered.

    Does stress delay a period?

    Yes. Stress, weight change, very hard exercise and some medicines can delay or suppress ovulation, and the period with it.