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.
Best Answer
'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:
The usage is not very productive:
........
'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':
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