Difficult babies with an intense and highly reactive temperament tend to be

Difficult babies with an intense and highly reactive temperament tend to beirritable and unpredictable.

If a baby is difficult, it means that it may be quite demanding to deal with it. The baby may cry all the time, be generally irritable and dislike anything that the parent does. It is also unpredictable, meaning that it cannot be predicted what the baby is going to behave like.