I think you can choose one of two ways to handle it in the moment. You can ignore it, roll your eyes, and walk away or you can stop, look at her, and say something like, "why would you say something like that?," "that was really rude," etc. It depends a lot on your goal. My MIL likes to say things like that but I choose option 2. She now chooses her words more carefully around me and saves her nastiness for her son. Lovely, isn't it? But he chooses the ignore method. However, she enjoys the dig and the response so I've removed some of her power. When someone is truly oblivious or mentally ill, however, your words may not have any effect on them. I'm sorry you're dealing with this.

My mom has always said things that were hurtful to me, in the guise of being "helpful" or thinking that it's just no big deal to insult me. Just a handful:
-I once dated a man who had children and didn't want more and I was conflicted on the relationship. She told me to stay with him because "I never thought of you as very maternal, anyway."
-She came to visit me 1 day after bringing my newborn home from the hospital. I'd made an effort to shower, fix my hair, and put on some make-up. I thought this was a pretty monumental effort. She said, "your hair looks as stringy as that girl I saw on the news this morning."
-When DH and I told her we were trying for a second child she asked why would we want another.
-in HS she came to see me in a chorale performance. Her one comment was "don't stand with your hands linked in front of you. It makes you look like you are holding up your stomach."
And so on and so forth. It really used to hurt my feelings and I'd wonder why she was so cruel. Now, I understand that she's just that way. She has her hang-ups about weight and always has projected them onto me. Our relationship now is cordial but distant. I visit her once a year and call maybe once a month. She's in assisted living and sometimes I feel guilty that I don't see her/communicate with her more, but then I remind myself that she's reaping what she sowed. I just try to do better with my kids and hope that we will have a warm, loving, and non-judgmental relationship when they are grown. There's nothing I can do with her now, so we just smile and roll our eyes when she drops her little nuggets.

My mom and I have a complicated relationship and we're doing better, mending fences. She is insecure about many things and I usually don't tell her good things about my life because she immediately responds with bragging/referencing something about her life. Sometimes she even makes up stories to "outdo" me. I have accepted this for the most part.

The thing that I am having a hard time with is hurtful things she says about my parenting. E.g. in talking about my very rambunctious and disobedient 3-year old, she will say, "When he spends time at my house, he is a different child, so good and sweet. Nothing like at his own house." I know I am being sensitive but it just hurts my feelings that the insinuation is my parenting is bad somehow.

How do you get past those jabs from an insecure mom?