A new baby is a big change for everyone in the house, but the transition from a three-person household to a four-person household may cause the most turmoil for the firstborn child. You might be excited about the potential for your child to have a lifetime playmate, but he or she may not feel the same way. The baby might be seen as an intruder who sucks Mommy's time away from big sister or brother. But there are lots of ways to ease your child into sibling-hood, and the sooner you start getting him or her used to the idea, the better. The ways siblings adapt to new babies depends on a lot - mostly, their age. If the child is 2 or younger, then the only explanation you really need while waiting for baby's arrival is that the baby is growing in Mommy's tummy and will be here soon. That will work for awhile, but as the baby's arrival grows closer, the child may feel anxiety as he or she can't fit on Mom's lap anymore or Mom needs extra rest.
Kids this age are reaching milestones like potty training and moving into a toddler bed - it's important not to rush them into these big-kid things before they're ready. Regression will be normal, however, when the baby arrives, and the child shouldn't be punished. Start making the child feel included before the baby arrives by allowing her to be involved in decisions like decorating the nursery and choosing clothing. Refer to the new arrival as "our baby" or "your sister/brother" so that the child feels a part of the baby's life. When the baby comes, allow the older sibling to help as much as she's willing - fetching diapers, choosing outfits for the baby and showing the baby his toys are good ways. It might be helpful to have a relative or sitter on hand to help with the baby to increase one-on-one time with the older sibling. Talk about the older sibling's babyhood - it helps her to understand how much time and attention are needed when there's a new baby, and it should help her to know that you spent just as much time with her when she was an infant.