'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