OP I don't think it matters that it was said two years ago.
The timing of it was terrible and I think that makes all the difference.
She made her comments at a time when you felt vulnerable and were struggling.
She may not have known that (and I suspect that the fact that she didn't points to you and her not having the best relationship as it was) but that is not really an excuse.
If you had been in a better place with your own feelings then you might have been able to let this go.
The fact that she hit you when you were down, with one of the issues that you were already struggling with, makes it all the harder to deal with and let go now.
I know, I've been there. My relationship with my PIL's has never been great but up until 2007 I would have called it amicable.
But then we lost our son to stillbirth and less than eleven months later we lost our daughter to prematurity and they made our lives hell.
They took the worst thing that had ever happened to us and treated us and our babies like shit.
My MIL argued with me about the time our son was born, even though she was not there at his birth. She asked when we were trying again, three days after he had been born. She caused an argument before his funeral because DH did not sound happy on the phone. They had tantrums on mothers day and fathers day because we didn't send them cards and couldn't understand why we didn't want to celebrate. They talked about my medical issues with strangers and came up with reasons why I had lost them that weren't true. She told me that our daughter didn't count as their first granddaughter because she was dead. She rang me on our stillborn sons first birthday, which was also six weeks from our daughters birth, to tell me how excited she was that her 'real' first granddaughter was being born by c-section the next day. She asked if our daughter had been born with all of her face or not. There's a whole essay of other things she has done and said to us somewhere on AIBU, so I won't put the rest here now.
I think any one of those things was a good reason never to speak to her again. But for two years I tried to put them behind me. Those things I have put here were spread out over those two years but the comment about our daughter's face was the final straw.
I told them I needed some space away from them (they then moved onto the next street to us) and instead of giving me that space they all but stalked me and made my life a misery. I felt like a prisoner in my own home. They used to park outside or drive past staring at the house. If I went out and they saw me then they would follow me. I would keep all the blinds and curtains closed so they couldn't see in.
I felt like I was going mad. We sold the house and moved but still they managed to drive me to panic attacks. The doctor says I am suffering from delayed, grief-related depression brought on by the stress they have caused me.
What I'm getting at, is that at the time when I was at my most vulnerable, these people were giving me an emotional kicking and it's going to take me a long time to get over it because I am still dealing with the grief and pain and ill health caused by losing my babies.
And yet my PILS can't see that or understand it.
So I can completely understand why your MIL, picking at you and chipping away at your confidence at a time when you were feeling vulnerable, could drive you to hate her two years later from one nasty comment too many.
It doesn't matter if she knew or didn't know how you were feeling. She's been a pregnant woman. Unless she has the skin of a rhino she must know that as well as being a fantastic experience it can be stressful and leave you feeling like an emotional wreck.