The following is an extract from the British Airways website
http://www.ba.com regarding their policy on newborn babies travelling with their mother:
"A baby must be at least 48 hours old before we can accept him/her on to one of our flights, subject to having been born without any complications, due to the physical effects of flying at high-altitude.
Therefore, please note that we would always advise the child's parents or guardians to ensure that the treating doctor agrees to him/her travelling with us.
Similarly, the baby's mother can be accepted for travel from 48 hours after the birth of her baby subject to having delivered without any complications or surgery.
If a caesarean section was performed or if surgery was needed for the mother then the minimum period before we could accept her for travel (with medical clearance from our Health Services) would be 10days.
However, this is also subject to approval for travel from her treating doctor."