If there are previous children, the moment to tell them that mom is pregnant and that a sibling is on the way may generate doubts.
It is convenient to tell them relatively early so that they can gradually assimilate everything that is coming. However, the 9-month wait can become very long for them and, in addition, if something goes wrong with the pregnancy, they will have to be given an explanation.
For this reason, many couples wait until the end of the first trimester to tell them, although this is a very personal decision.
The way to do it will depend on their age. Telling them stories of your own birth or visiting friends who have recently had a baby can help them understand, but it is difficult for them to appreciate all that the arrival of a sibling will mean, especially if it is the first child.
It can also be a good idea to involve them so that they feel involved and to buy and prepare with them the things for the baby's arrival, let them participate in the choice of the name...