It's perfectly normal for a baby to be wary of unfamiliar faces or friends and family members she doesn't see all the time. This tends to crop up around 9 months and may last until about 18 months, by which time many babies become more comfortable with new faces.
This stranger anxiety is caused by your baby's realization that there's a difference between Mom and Dad and everyone else -- and she'd rather be with who she knows, loves, and trusts. (Take it as a compliment!) Stranger anxiety is also related, development-wise, to separation anxiety -- your baby's ability to understand that you're still around even when she can't see or be with you. The result? Your baby cries any time you leave her or hand her off to a friend or relative.
Separation and stranger anxiety tend to come and go until your baby's verbal enough to comfortably communicate with people besides her parents. A certain amount of this behavior is inevitable, but leaving your baby with a capable, nurturing sitter for short stints (an hour or two at first) will help her adjust to new faces and life away from you. She will get upset when you go, but over time she'll see that you always come back, and that she can feel safe and have fun with other people too.
Copyright 2009 Meredith Corporation.