grammar - How to deal with quoting a grammatical error?

What should you do if you’re quoting someone, and that quote has a grammatical error?

Say for example that I’m quoting this line from the American Pregnancy Association:

The term used for a pregnancy that ends on it’s own, within the first 20 weeks of gestation.

The proper way to write this is without the apostrophe. I know that, and you know that, but the Association’s proofreader apparently missed it.

The question is what I should do now: how can I quote this without sounding like I’m the one committing a grammatical error?

Answer

Use the word "sic", which is Latin for "thus". It indicates that the error was in the source material. But beware - it can be considered rude. I would quote the passage thusly:

the term used for a pregnancy that
ends on it's [sic] own, within the
first 20 weeks of gestation.