My mum is having a baby. I'm happy for mum. I hope my mum has a baby girl.
There are three instances of the word mum in the example. I thought the first and third did not need capital letters as they as simple nouns, used in the same way as the word 'mother'. I corrected the second sentence to read: 'I'm happy for Mum' because it's a pronoun.
I was told that all instances of the word needed to be capitalized because 'mum' is abbreviated. What do you think?