I've seen a few people on other websites commenting that Mary Margaret was rude, pointing out that not everyone who takes a test wants it to be positive. But I'd have interpreted that "good luck" as a more open "I hope the result is what you want" rather than "I hope it's positive". I found the prison guard's comment to be more rude and insensitive. Emma was 17, in prison, maybe the guard was even aware of her family circumstances meaning that there was no one to help her with a child or look after the baby while she served the rest of her sentence. The pregnancy obviously wasn't good news at that moment.

^ Pretty much this.