Hi. How old must a baby be before it can fly?

Can anyone tell me how old a baby needs to be before it is allowed to fly? I am asking this as I am pregnant with my first child. My husband and I got married last year, after being together 12 years, we are both 29. My mum lives in New Zealand with my step dad, and my father ped away of terminal cancer Feb 2007.....now my mum can't afford to come home for the birth of the baby, as they came back for 6 weeks last year to celebrate and help plan our wedding, and then they are back again October 2007 for two weeks as my step brother is getting married. Before the pregnancy we had planned to visit my mum in Feb 2008, and she is really excited about it, haven't been over since she emigrated so everyone wants to meet us, now I am pregnant and Due 23rd Decemember, I am very dubious as to wether or not we should continue to plan to visit. Obviosuly I will want to see my mum, and they will want to meet their grandchild, but i don't know if it is safe to fly that early. Any advice? x