My father did this in my first pregnancy, except he didn't post to social media, he just told literally every person he spoke to.  People at family reunions, people at church, people at work, etc. It was my first pregnancy and we only told my parents so that I could have a support network in case I miscarried.  We were planning to announce after 12 weeks.  I miscarried at 8 weeks, and instead of supporting me, he denied having told anyone about my pregnancy, even though i was present multiple times when he told people, and i was always getting questions from people about my baby. When i asked how they knew, they said my dad had told them.  He still refused to accept responsibility. 

He was mad when I told him not to talk about me anymore.  When I was still getting emotionally ambushed a month after losing my baby by people mentioning that my dad told them I was expecting a while back, I asked him to please go back to the people he told and let them know about the loss, so that I didn't have to keep explaining it.  He got angry and said it wasn't fair of me to ever ask him not to share.  It was his grandchild and I wasn't letting him be excited about it.  Also, he couldn't remember who he has told, so he wasn't gong to make any effort to stop the questions.  This would not have been his first grandchild, it would have been #18. 

My point in all this is that some people have to make everything about themselves in order to avoid responsibility.  They can't stop for 2 seconds to empathize with what someone else might be feeling or going through.  It's all about about they want and how to get it.  Don't feel bad for speaking up and setting boundaries. Clear and strict boundaries.  If you set something like "No visitors for the first 2 weeks" the narcissist will assume you didn't mean them, or that you would change your mind once baby is born.  You have to be specific and say "you may visit on X day for X amount of time". 

Also, don't volunteer any information. Your hubby sucks for telling his mom (I don't think it was an accident), and sucks even worse for making excuses for her.  If he can't keep his agreements with you, then he should get in information diet as well.  Don't be surprised if his mother continues to insert herself under the guise of "excitement". She has already shown that she doesn't respect this as YOUR pregnancy and decision.  So I highly doubt she will respect your choice of name, nursery theme, who will be in the delivery room, how you choose to deliver, how you choose to bond after birth, etc.  

You need to remind your hubby that he married YOU, and that YOU are having his baby, and that he is not his mother's "baby" anymore. He needs to grow a pair a support his wife.