Many parents feel it is a good idea to wait until the end of the first trimester, when the chances of having a miscarriage significantly go down, to tell their kids that they are expecting another baby. They prefer to spare their children from the grief that would occur if they were to lose the pregnancy. This is totally understandable, but can prove tricky if you are suffering from a lot of morning sickness and your kids are wondering why you are feeling so ill. When your kids know that you are, in fact, pregnant and not ill, it can save a lot of trouble as well as worries on their part.
Gaining a new brother or sister is a wonderful event, but one that does take a lot of adjustment. Some kids prefer knowing as soon as possible that they will no longer be the only focus of attention in a family, and they also love helping to prepare for a new baby they like to come shopping for clothes, and want to help to decorate the nursery. Really small kids might not grasp the concept that well, and when you tell them the baby will be born many months later, they might have difficulty understanding what that really means. That is why some parents like to wait until signs of the pregnancy become visible (a baby bump), so that the concept becomes a bit more real. Also see, would you allow your older children to watch a new sibling's birth?