It's very common to hear women say "I'm on my period" or "I'm having my period".

Can somebody please confirm to me as to whether these two phrases are grammatically correct, and if they are, WHY they are deemed so? The actual definition of "period", according to Oxford Learner's is "the flow of blood", however, if you were to replace "period" with "flow of blood", the above two phrases (or rather the first) do not make sense.

  • 2
    The first is more BrE and the second is AmE. It's just the way it is. They are idiomatic expressions. You can't, obviously, just delete and replace these terms with flow of blood. That is an explanation; not a synonym.
    – Lambie
    Aug 2, 2021 at 18:34
  • The 'actual definition' is the period of time during which the woman is menstruating. The explanation in OLD is a deliberately simplified one. Aug 2, 2021 at 18:43
  • Thank you, @Lambie. You're right. Aug 2, 2021 at 19:26

2 Answers 2


Grammatically fine, "on my period" has the form of a prepositional phrase "preposition + (determiner+noun)".

Idiomatically correct too. Idiom is determined by use and this is the phrase that is used. Whereas "on my flow of blood" is not idiomatic. As Lambie says "They are idiomatic expressions. You can't just delete and replace these terms with "flow of blood". That is an explanation; not a synonym."

You could investigate how this idiom developed over time. It would have to be a fairly recent idiom since "period" in the sense of menstruation dates back to 1830. The word "period" refers to a length of time, which points to a likely development of sense:

We do tend to say "On Tuesday" or "On the weekend" with a period of time lasting a day or a few days. So "On my period of menstruation" would seem to be a natural generalisation, which would be shortened to "On my period".

  • Thank you, @jamesK and everyone else. I rather dislike the phrase, and I myself refrain from using it, but this question has been bugging me for a while now. Perhaps you could kindly clarify something else - is the noun "period" referring to the period of time of menstruation, or to the actual blood? Aug 2, 2021 at 19:21
  • It could refer to the time, to the flow or to the blood itself. "My period was heavy this month" is about the rate of flow. "Her period leaked onto her new jeans" refers to the literal blood. But the etymology links it to "the period of menstruation" it is probably a euphemism.
    – James K
    Aug 2, 2021 at 19:30

There is no reason why on my period have having my period would be grammatically incorrect. Compare I'm on my first cup of coffee or I'm having my party.

On is a very flexible preposition and one of its uses is to indicate something like being in the process of completing or working on:

I'm on call
I'm on duty
I'm on the first question on the exam
I'm on the last page of the book

Having can have a very similar meaning:

I'm having breakfast
I'm having a party
I'm having a panic attack

  • Thank you, @Juhasz Aug 2, 2021 at 19:25
  • I don't know if delicacy should prevent me from mentioning that the on the form is very common in at least UK casual usage, rag and blob spring to mind. Aug 3, 2021 at 7:23
  • @MichaelHarvey I'd say it's so common now that people will just say "I'm on". Although when I was younger, we "had the curse"! Aug 3, 2021 at 13:54
  • Even 50 years ago girls used to say "I'm on", "I came on", "I was late coming on", "I haven't come on!" etc. Aug 3, 2021 at 15:01

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