Mummy vs mamma

Mummy

name
  • One's mother. 

mamma

noun
  • The milk-secreting organ of female humans and other mammals which includes the mammary gland and the nipple or teat; a breast; an udder. 

  • An accessory cloud like a mammary in appearance, which can form on the underside of most cloud genera. 

How often have the words Mummy and mamma occurred in a corpus of books? (source: Google Ngram Viewer )