I had this many years ago in a group of mums. Looking back, they were not real friends but it did hurt at the time as I had hope that we and the children would be in touch for many years.
When I had my second child, one of the mums started being really odd with me, like not replying to my texts, then after a long time, inviting me to do something or asking for childcare. She didn't even get me a present when my second one was born, then gave me something when we met at an another's mum's house for the birth of her new child as it would have been quite embarrassing to give the other mum something and not me. It felt like she was trying to snub me at every opportunity.
I also noticed that a couple of the other mums were a bit odd too. It was all quite hurtful and uncomfortable. I think three of them were definitely talking about me behind my back.
I only kept in touch with one mum from that group, who was a nicer person and I think felt left out too. Looking back, it was just a period of my life and those people were not meant to stay in my life.
When I moved to another area, I made sure I cut all ties with them. I was not interested in playing silly games with them. Unbelievably that people behave like as adults. My advice would be to try to stay in touch with the ones in the group who you feel closer to and who are more decent people and forget about the rest.