mother at birth of her child is extreemely happy. this is how the word was made. mother at birth = mirth.
just try to visualise the ramayan kind of scenes where the demons say "My Earth" [mi-rth] and laugh in a similar fashion.
mirth sounds close to myth..which are usually unbelievable and hilarious and makes us laugh
mithr.. friend .. merriment and laughter
MIRTH=MARY+BIRTH. For Christians, the BIRTH of Christ to MARY is cause for MIRTH.
mirth = Mother + birth; Mother has given the birth of a child so she is happy.