Having a new baby is very exciting, even if it isn't your first child. Along with the regular preparations for a new baby come the added responsibilities of preparing other children for a new bundle of joy.
When to you tell your children and what to say about the pregnancy and a new baby really depends a lot on the age and maturity level of that child.
An older child with more of a concept of time passage will be able to learn earlier in the pregnancy than a child who has no concept of time. Some people choose to relate the time of the baby's birth to an event, which works well even for smaller children. For example, "The new baby will come near Thanksgiving."
Another thing to keep in mind about when to tell a child is how fearful you are of having to explain an early loss.
We actually told our older children early in pregnancy. My personal view point was that we'd have to explain my elation/nausea or my depression. When we did lose our baby, we simply explained that our baby was born way too early to survive outside. They still remember and think about our "purple baby," as they named it.
Most children have gathered a concept of pregnancy from pregnant relatives, friends, and people that they see on the streets. An older child may be really interested in the mechanics of pregnancy, such as the placenta and how it feeds the baby. How a baby learns to breathe, eat, or eliminate waste. A few children ask in passing how the baby got there. Remember to give them details of only what they really want to know. A few questions from you might help you clarify that, and they may not really be asking for a complete description. Sometimes they just want to make sure it isn't really from watermelon seeds!