Separation anxiety in preschooler: Is this normal?

Q.

I'm a stay-at-home mom with children ages 2, 4 and 10 years old. My middle child has always been very attached to me. But recently he has been showing excessive separation anxiety. If I leave the room, he runs after me asking, "Mom, where are you?" We occasionally have a sitter. But I rarely am not with him. Recently he started preschool and is doing well there. But he has begun attaching to his teacher. How can I help him overcome his anxiety? I want him to feel secure, but I also need some breathing room.
Suzanne / Missouri
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Mayo Clinic pediatrician Jay Hoecker, M.D., and colleagues answer select questions from readers.