"Here, for instance, was a child who had so far been the only one; and now he was told that the stork had brought a new baby. He looked the new arrival up and down and then declared decisively: The stork can take him away again!"
This story — one you'll almost definitely have heard before in some form — does a good job of describing what parents worry about when they're about to have a second child. Versions of this story are so common that you'll probably be infected by the worry that your firstborn will be angry, sad, jealous, or confused when their new baby brother or sister arrives. Indeed, studies show that mothers are deeply concerned about the "impending disruption, experience guilt and sadness over the loss of their relationship with the firstborn", and "may question their ability to cope with the older children’s misbehaviors once the baby has been brought home". 
Do Young Children Really Experience The Birth Of A Sibling As Dreadfully Upsetting?
They may. Anecdotally, I have certainly heard from friends whose firstborns wished that "the baby could go away again now", who suddenly wished to start nursing again, or who actually hit their baby siblings. I am sure you have too — and between the two of us, we can establish that the arrival of a baby sibling can be quite turbulent. It doesn't have to be, though, and a very comprehensive review of studies on the topic of firstborn children adjusting to siblinghood came to a much more optimistic conclusion.
"The current review did not find strong support for the notion that firstborn children consistently display clear increases in disruptive behavior and adjustment difficulties after the birth of an infant sibling," its authors wrote. "Instead, there was evidence of some disruption, some growth, and no change at all in children’s adjustment across the transition to siblinghood. Children vary widely in whether they are or are not distressed by the arrival of a newborn sibling." 
You don't need to start with the assumption that your older child will struggle, then, but keep in mind that:
- People in general don't like feeling excluded, and social exclusion makes it much more likely that they'll behave in ways you won't enjoy . If you're so busy with the new baby that you don't pay any attention to your first child, who previously had all the attention, they are, of course, going to feel all sorts of things, from anger to grief, and from jealousy to depression.
- Children thrive when they feel loved, secure, and cared for. Though stressful situations are inevitable, child-rearing practices that help children cope with change, stress, fears, frustration and disappointment help them grow into happy and well-adjusted adults. 
- Plenty of adults find it quite hard to cope with all kinds of change! The ability to deal with stressful events is still very much developing in early childhood, and small ones need our support to cope and then thrive . Most firstborn children will experience the transition to siblinghood somewhere between the ages of two and three , at a time when they're very much not done learning to deal with stress by themselves.
How Can You Help Your Firstborn Adjust To Their New Sibling?
Using the three ideas established above — kids like to feel included and not excluded, they thrive when they know they're loved and cared for, but they may find it hard to cope with change — as a guide, here are some things parents can do to help their child embrace being a big brother or sister.
On the coping with change front, make any big changes you're going to make well before your new baby is born. If your child is currently sleeping in your bedroom but you'd like them to transition to their own, for example, begin now. Even if the process isn't complete when their baby sibling is born, your child will be less likely to associate the change with the baby.
You can further help your child cope with the change the baby will bring by making sure they'll know what to expect. Read books (plenty to choose from!) and watch videos about babies, go visit some newborns if you can, and talk about what to expect. This will also make them feel included in the process. Other things you can do to help your firstborn feel included in your family expansion project are:
- Take your child along to some of your ultrasound appointments so they can see the growing baby. Your child can start feeling connected to their sibling by hearing their heartbeat, seeing their head and body, and maybe they can even recognize some feet or hands!
- Ask your child to help you pick out some clothes and bedding for the new baby, and maybe even ask for their input on how to decorate the nursery. If you're especially brave, ask for name suggestions.
- Once the baby is born, your firstborn can feel included and be quite the help by bringing you diapers, baby wipes, or picking out a new outfit. (Yeah, I know — that sounds like a chore rather than a privilege! My toddler enjoyed this, though, and children generally feel happy knowing they can contribute something valuable to their families.)
Newborns — though certainly more straightforward than the children they grow into — are, of course, a lot of work, not to mention the fact that they literally keep you up at night. How can you make your first child feel loved and secure during this, frankly, crazy time? This is going to sound silly, but my beloved baby carrier might have been my number one help! (See: How babyewearing can make your life easier.) It allowed me to have my hands free, in the literal sense, and that meant being able to hold my daughter's hands on walks and to turn the pages of a book. You can also use the time your baby is asleep to do special things with your older child, like crafting and cooking — but also to hug, tickle, or play horse with them. Your older child won't stop craving your physical touch, after all.