You have to be as positive as possible.
As gentle as possible....Say that in a few months ''something wonderful is going to happen to the family'', that you all have been given a miracle of life.
If it is told in a exciting way, because it is a new experience...that you all will be very happy....
Say that you can go out and buy a few little things for the new baby.If you are asked 'how' the baby got into Mummy's tummy....unless the child is over 12, just say it is one of life's biggest joys...that happen at times.
If the child is 12 and over, your on your own! lol that's just another joy for you as parents to get through :) (Use Google for ways to express it, that come close to your lifestyle.) but be very careful....Some children can find that truth, a little hard to except.