English Translation
My sister is pregnant with her first child.
Literally, “Mi sister is pregnant of her first son/ child.” I know most of us wouldn’t infer that anything inappropriate happened to bring about this pregnancy. But I was curious about what’s going on so that Spanish speakers don’t ever seem to worry. So, I played around with some translation software to see what differences I could tease out for the preposition ‘de’ in this sentence. If you replace ‘de’ with ‘con,’ you get the same English sentence. The problem being, the meaning’s not the same, as ‘con’ conveys the sense of ‘alongside.’ Now she and her first child are both pregnant, but that’s not what the original sentence means!
The original sentence seems to use ‘de’ much as in a phrase like, ‘Llené el pozo de concreto’ (“I filled the pit with concrete”). I see ‘llenar de’ often enough, but maybe it’s not a set phrase like I was thinking. The best way I can explain is, the particle is serving to tag supplementary information. Which also implies that ‘Llené el pozo con concreto’ means the concrete helped me fill the pit the way a friend or coworker might. Fun!