You can contract "is" and "has" when they are auxiliary verbs to 's
You can contact "are" to "'re" and "am" to "'m"
You can contract "have" when it is an auxiliary verb to 've
You can contract "had" and "would" (auxiliary) to 'd.
You can contract "will" and &...
There are standard contractions and there are contractions used by writers in dialogue:
So, contractions and tags like the ones below, are standard:
He must have left early, mustn't he? [standard]
He mustn't have liked them much, must he? [the tag here is less usual]
They speak Hindi, don't they? [standard]
They should have done the work better, shouldn't ...
Most informal contractions like these will be written down when the writer is conveying spoken English. Then anything the reader can understand is acceptable.
In your example you want
but I don't see how that's relevant
The way you wrote it the contracted "is" is in the wrong place.
For clarity, "pregnant with..." usually refers to the unborn baby. If referring to the other parent, we say "pregnant by...".
Assuming you understand this, "I'm pregnant with nobody" isn't good grammar, strictly speaking. You would simply say "I'm not pregnant".
However, there can be contexts in which we may use that ...