their, thier, there, or they're?

I know I should just quickly look it up in an online dictionary, but the word that indicates plural possessive is "their" or "thier"? Both look misspelled to me. I blame my pregancy that I even noticed such a small thing and then forgot how to spell such a small thing.

I do know the difference between "there", "their/thier", and "they're" and all the stuff about proper usage. I didn't forget everything about 9th grade English or other things I needed to know to major in the subject. But I did forget the spelling rules for "i" before "e" except after "c". Good Grief Charlie Brown!