You say you work with this woman, not that she is a close friend, so why are you involving yourself in this?
I doubt any woman in this day and age actually lives through all nine months of a pregnancy without realising that she is going to have a baby.
So either she is a pathological liar and there wasn't a baby last year, or poor soul, she felt forced to give the child up for adoption, or had a severely handicapped child that she didn't have the strength to take home and placed in care. I imagine you can still do this with hideously handicapped children?
Your manager had absolutely no business telling her staff any of all this.
You can and should ignore what you have been told of your work-mate's private business and merely congratulate her on the birth of her baby when it arrives.
In your place instead of concerning myself about your work-mate's problematical motherhood, I would start looking for a new job, somewhere where the manager is capable of not gossiping about her staff. How do you know this is not a pack of lies, or that the manager is not spreading damaging and untrue gossip about you?