How does your husband bring this to you? Is he just feeling the pressure from his mother, or does he really think that you guys should be doing more?
First, find out where he is, so you know exactly what you can expect when you do the next thing. If he's not going to say it to his mother RIGHT NOW (which he probably won't since he hasn't already), then you might have to find an opportunity to bring it up. Surely, she shows disapproval on some level in your presence. When that happens, tie it in and tell her that you realize that her (your daughter's) daddy was doing diffrent things at her age, but she is not the same person as her daddy, and you guys have different emphases in your parenting goals. I use that term a lot--"parenting goals"--because it says that I do have a plan and am not just winging it. Nobody else has to know your specific goals for your children. Just express that she is right on track for what you want for her and that you are encouraged every day when you actually see it coming together. You can even tell her something like, "Yes, we plan to do that (whatever she is suggesting) at XYZ point." Then, take what you want from her suggestions and incorporate it.
I get the whole hold-it-in-until-you-explode thing. I can be the same way, and that does nobody any good. Start small telling people what's on your mind. Let your husband in on it, though. I don't know what your relationship is like and if your keeping your mouth shut is part of your charm; you don't want to suddenly be loud-mouthed and opinionated and throw him into a tailspin. This issue is more complicated than MIL being critical. Use being pregnant and sensitive to open the door to this discussion with your husband, but let him in on your desire to modify this closed-mouthed part of your personality (Do you have that desire?), and ask for his assistance and guidance. That way, he can feel like he's got some say, while you work out the kinks.
Good luck!