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From my understanding both "5 weeks pregnant" and "5 week pregnancy" are grammatically correct but I don't know when to use one instead of the other and which one sounds more natural for native speakers.

For example: I would say "She is 5 weeks pregnant" but not "she is 5 week pregnancy." The first one just sounds right to me but I don't know any grammatical rules behind it.

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'She is five weeks pregnant' is (a) very commonly used but (b) a rather unusual type of example of a common construction: a measure phrase which includes the premodification of an acceptable adjective. See, for example, Norbert Corver: Getting the (syntactic) measure of Measure Phrases.

Other examples are:

This piece of wood is two metres long.

The mountain is three miles high.

The book was five weeks overdue.

The train was 50 minutes late/early.

The usage is not very productive:

*He is two weeks ill.

*I am two hours busy.

*The bath was 10 gallons full.

*The liquid was 20 degrees hot.

??He was five weeks overdue.

........

'She is 5 week pregnancy.' is unacceptable.

A 'five-week pregnancy' usually means that the whole pregnancy lasts 5 weeks (a hedgehog? but 'gestation period is normally used instead), though it can be used for 'the state of a lady who has been pregnant for five weeks':

Is it normal for a five week pregnancy... to look like a four month bump?

[babycenter community]

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  • +1 for The usage is not very productive (but I still think the question belongs on ELU, where that kind of information is much more relevant). – FumbleFingers Feb 2 '16 at 13:14
  • On day 35, the fetus is a "five week pregnancy". By law in many states you cannot elect to terminate a pregnancy of eighteen weeks or more. What's wrong with this usage? – user662852 Feb 2 '16 at 14:58
  • @ user662852 Please read more carefully. I say 'She is five weeks pregnancy' (suggested by OP) is unacceptable, not 'a five weeks pregnancy'. I even include an example of the latter. – Edwin Ashworth Feb 2 '16 at 20:32
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5 weeks pregnant

Is saying she's pregnant and is 5 weeks along. The implication is that the pregnancy will continue for a further period of time.

5-week pregnancy

Is implying that the entire pregnancy was completed in 5 weeks (which is obviously silly).

The main difference here (though not a grammatical rule as such) is that you'd use "pregnancy" to refer to the entire term of being pregnant rather than as some point between the beginning and end.

Relating more to your sample sentences, "pregnant" is an adjective, while "pregnancy" is a noun, which is why your first one is grammatically correct while the second is not.

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  • Nothing silly about a five-week pregnancy if you are talking about rabbits or squirrels. – Brian Donovan Feb 2 '16 at 13:08
  • That is a good point. – John Clifford Feb 2 '16 at 13:16
  • "In some states you cannot terminate an eighteen week pregnancy" - any issue? – user662852 Feb 2 '16 at 14:59
  • None whatsoever. :) already conceded that it's not as silly as I initially thought as there are usages I hadn't considered. Though honestly my personal preference would be to say "In some states you cannot terminate a pregnancy at eighteen weeks" for the reasons I stated. – John Clifford Feb 2 '16 at 15:00
  • Silly, or sad. It sounds like a miscarriage to me. – Martin Bonner supports Monica Nov 29 '18 at 17:12

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