I often talk to people who have just had a baby, and their news usually includes something along the lines of:
It was a boy / girl. The name is ________. He / she weighed _____ pounds _____ ounces.
Usually I reply with,
"Congratulations, you must be so happy", but I've noticed that some people address the weight of the newborn in their response specifically.
Looking on Wikipedia:
The average birth weight in babies of European heritage is 3.5 kilograms (7.7 lb), though the range of normal is between 2.5 kilograms (5.5 lb) and 5 kilograms (11 lb) (all but 5% of newborns will fall into this range). Babies of south Asian and Chinese heritage weigh about 240 grams (0.53 lb) less.
What are you supposed to say if the baby is over / underweight? What is considered over / underweight?
Is commenting on the baby's physical characteristics appropriate? If so, how does one address this tactfully?