This happened a couple of months ago, but I've really just been a lurker during this pregancy.
My mom lives several states away, and we don't have the best relationship. When talking about her visit for when I have the baby, I politely told her that a visit of a long weekend would be perfect. I used the excuse that we're not hosting anyone since it's a second pregnancy (we're not, but my step-mom and MIL are local and will probably be by pretty often to lend a hand) and that we need time to ourselves for our family to adjust to the new addition. Really, a weekend is perfect because my brother can come visit too and be a great buffer for her craziness, plus my mom may actually be useful in helping to entertain DD#1.
At first, she threw a fit. She told me she couldn't understand why I didn't want her for a lot longer as she was "such a big help" with holding and rocking DD#1 last time (yep--she actually used that as proof of how helpful she is--during the last post-baby visit, she would say things like, "Oh, it's dinner time--why don't I hold the baby so you can cook?"). She also told me that it's not my fault that I'm not remembering how great she was, because I was so hormonal.
I actually laughed when she tried to remind me how helpful she was. Then I calmly told her that if that if my hormones were truly to blame, I will be equally hormonal this time, and a long weekend works best for our family.
I'm glad she backed down without picking an even bigger fight--and perhaps I can actually hold my "hormones" in check for a weekend with her.