It sounds like your step-son has been through a lot and you are probably his best chance of having a positive loving relationship with a mother figure. Being pregnant and becoming a biological parent does not mean you can’t be a terrific step-mom to him.
Preparing him for what is going to happen is the first step. I recommend reading books that will help him know what life is going to be like with a baby in the house. I particularly like Heidi Murkoff’s book What to Expect When the New Baby Comes Home for this age group. It can also be very helpful to make a book of your own with him that outlines the way things are right now -- the new changes and the way things will continue to be the same (i.e. daddy will still give you a bath, we will still read you a story together every night, etc.). These books are easy to make on photo sharing site like Kodak EasyShare or Snapfish.
When you return from the hospital, I recommend that your husband not have the new baby in his arms when he first walks in the door. Let him have his arms available for hugs with his son. Make sure that your husband has one-on-one time alone with his son every day, even if it is just for 10 minutes, and try to do the same. You may be surprised at how happy you are to hand off the baby to your husband so you can spend some time with your step-son.
Be prepared for your step-son to act out some and make sure that you never leave him alone with the baby. The more inclusive you are with your step-son, the better things are likely to go. Give him small tasks to do like bringing you a new diaper, pick out the baby’s clothes, or push the stroller. But above and beyond everything, you and his father let him know that you love him just as much as you did before the baby arrived.
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