I have an aunt Maryrose, love her name. We sometimes call her Rosie or MR but mostly Maryrose. The name just sounds so lovely so we use her full name. I am not sure if it is true but I heard when I was younger that she was Christened Mary but decided in childhood that her name was too boring for her (she is an artist so I guess wants to be different ) and she renamed herself Maryrose. I will have to check if that is true!
As another idea, I also have an Aunt (-in law) by the name of Mary Louise. She was known as Marylou or just Lou.
I guess in my family, Mary does not hold much weight and the name is always dressed up a little
It is worth considering nicknames with this though. It seems if you lengthen Mary in some way, you are likely to get a nickname which drops the Mary and focuses on the other name. Would that annoy you if you really wanted the name Mary to be the dominant name? I feel that using just Mary might lead to the nickname 'Mare' or 'Mares' in that Aussie way that the ending is dropped or replaced. worth considering?
I say go with your gut - if you both love Mary, this little girl is meant to be a Mary. I think it sounds lovely - especially with Charlotte, and with your other girls' names Good luck with the new arrival
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