One basic principle of semantics is that you don't mean more than one thing at once; if words are polysemic, their use in a given context compels the hearer/reader to pick up one among the many possible meanings in the lexicon instead of adding them.
Another related principle is that the exocentric, or global, sense of a stereotyped expression holds priority over the endocentric meanings you could form by breaking it into its verbal components.
Let's take the expression "to be with child". English usage confers it one exocentric meaning (= "to be pregnant") which rules out the possible endocentric meanings a foreign hearer/reader could mistakenly read into it (e.g., to be somewhere with a child around).
Exegesis, especially in prosaic genre, is generally safe when it sticks to the above semantic principles. Whenever the exocentric meaning of an expression is attested in a similar linguistic context (as en gastri ekhein in the LXX) and there is no contextual indication that anything else is meant, why look for complications?
Of course poetry, ciphered speech, or the mere play of intertextuality within Biblical and contemporary literature often lead us to outstep those basic principles of semantics in order to find a superposition of meanings. Still, as far as exegesis (vs. creative reading) is concerned, there must be some contextual indication that such superposition of meanings is in order. I fail to see this in the extant text of Matthew 1. To me the meaning "to be pregnant" is fully satisfying.
Yet a mistaken interpretation of a text is itself another text which may be worth its own exegesis, and generate further creative readings... but here we are out of the scope of traditional Bible exegesis.