Whatever - it's her day!

Do you call your female parent Mum or Mom?  A company that produces greetings cards has decided to go for Mum this year, maintaining that Mom is American.  But is it that simple?  I live in the West Midlands, and apparently Mom is commonplace round here - even though I personally say Mum.  You could argue that as mother starts "mo", Mom is more logical, or you could simply go with your gut - and for me, the "u" sound is more cuddly, more loving.