Why It’s Bridget Jones’s (not Jones’) Baby
I almost forgot that the movie Bridget Jones’s Baby is coming out this weekend until my friend Stephanie asked whether it would be Jones’ Baby, or Jones’s Baby, or maybe Joneses’ Baby.
‘Jones’ Is Singular
Since it's just Bridget, she is a single Jones. It is the baby of Bridget Jones. We aren't talking about a family of plural Joneses.
How to Make ‘Jones’ Possessive
You can properly make Jones possessive two different ways. (A lot of people were taught one way or the other, but it’s actually a style choice.)
If you’re following Associated Press style, you add just a lone apostrophe after the S, but if you’re following the Chicago Manual of Style, you add an apostrophe and an S after Jones to make it possessive.
It's Bridget Jones's Baby and not Bridget Jones' Baby because the studio decided to go with Chicago style.
Looking at the official name of the movie on the posters, it looks like the filmmakers decided to go with Chicago style because her name is written Jones’s: Bridget Jones’s Baby. So that's the only reason it's Bridget Jones's Baby and not Bridget Jones' Baby—the studio decided to go with Chicago style.
That’s your Quick and Dirty Tip: You can make names that end with S possessive two different ways—with just an apostrophe or with an apostrophe and another S—it just depends on which style guide you follow, so be sure to look it up.