Grammar – How to Use Embedded Quotations?

Square brackets are used in quotes to mark information that was not in the original quote. This applies equally to added words and omitted words.

Compare

I wonder... who did that?

and

I wonder [...] who did that?

In the first, the speaker is pondering something; the question is somewhat rhetorical. In the second, the question is literal.

Edit: yes, you can use this multiple times in a quotation. Just be careful not to leave out so much that the quote becomes incomprehensible, or worse, changes meaning.

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.