Choose a quiet time to talk to your child. You could say, for instance:
"Mom is expecting a baby: mom's tummy is growing, and so is her hear and that of daddy too. Therefore, in the heart of mom and dad in the heart, there's room for you and even more room for the little brother or sister.
The more children there are, the more love grows between the parents, so that each child is always much love for his father and his mother. Your parents love you forever."