Choosing Hope-
Congrats on your soon to arrive little one

I think your feelings are very normal. I don't think we stop missing a mother. As adults we function quite well on our own, but that wish for connection can happen at different times. I have noticed particularly at milestones and holidays.
This is a major milestone- you are becoming a mother yourself, and it's normal to long for that connection with a mother.
One thing I try to keep in mind is that, while I may miss having a mother at times, that connection with my own mother isn't possible, not because of anything I did or didn't do but because she isn't emotionally capable of making that connection - not with me, not with anyone - due to her emotional issues.
I have noticed grief can take a cyclic route. It gets better, but it's normal to be reminded at times. After my father passed away, the grief was more intense, but time to time, I miss him. Even if it's a complicated relationship, it's normal to miss a significant person.
With my mother, it's odd to grieve a person who is still living, but I also do that at times. While we are not NC, even when I do speak to her, or see her, I am sometimes shocked at the disconnect. I think our relationship may have felt closer when there was more drama and more co-dependency on my part but now that I have decreased these behaviors, there's an emptiness to it.
I think your letting your parents know you are expecting is probably the better route as they'd find out from other people and this way, they did find out from you. It is possible they may come around and contact you, but this is where you need to consider your boundaries, so best to discuss this with your H about what they will be.
Also keep in mind that your longing for a mother might be a longing for the mother you wish you have, not the mother you do have. It might be wishing your mother was a different person than she actually is.
I recently attended a funeral for an older person in our community who passed away. Her children spoke about her. One thing that struck me as different was how they spoke about her. In my mother's family of origin, the discussions about family members seem to focus on their accomplishments. There's not much sharing feelings or feeling connected. In this ceremony they talked about the little things. "Mother always came to my sporting events". "Mother liked to bake cookies for us". "Mother loved teaching Sunday school".
Of course people only say good things at eulogies, but these were genuine- and reflected a loving and caring mother and the person that mother was. I realize that I don't have these "connection moments" with my mother. I do have them with my father. I can recall when he took us to the zoo, or played a game with us when we were little. My mother doesn't emotionally connect with us like this, and listening to her family - this may be a pattern with them.
When I say I miss my mother, I think what I am saying is that I miss having a connection to a mother. Right now, you are pregnant and tired. You will probably be tired a lot with a newborn. It's normal to long for a mother to come help, to hear" honey, take a nap, I'll hold the baby so you can get some rest".
But that may not be your mother.
What to do? Self care is key. It's OK to get help sometimes from other people. Having a newborn takes a lot of time. Perhaps there are things you can do to lighten the other load- get some prepared meals, ask a trusted friend to help with the baby so you can nap. I know with Covid it might be a bit more challenging to have help, but if there's anyone in your circle who you trust, even to bring over food, it's OK to ask.
It's OK to feel your feelings- whatever they are. While you may expect this time to be uncomplicated, it's a milestone. Grief can show itself at milestones. My mother is elderly now, and it would be the "normal" to help her more now. But during visits, she's emotionally and verbally abusive, and also not cooperative and so my attempts to help don't work. She actually does better with people who are not related to her. I think connection is the small acts of caring between people. With a disordered person, it might not be possible to have these moments. However, it is possible between you and other people in your life, and with this new baby.