It sounds like baby may be experiencing the beginning of separation anxiety. Some parents are frustrated when this stage happens, but it's completely normal and truly a wonderful milestone in baby's development. Separation anxiety means baby has the cognitive ability to understand that you exist, even when he can't see you.
Baby will eventually become more comfortable with his or her newfound awareness. In the meantime, here are some ways you can teach him or her that when mommy or daddy leaves, they always comes back—whether it's daytime or nighttime.
First, give baby opportunities to practice separating from you during the day. Start with playing extended versions of peek-a-boo with him or her. Disappear from his or her sight for a few seconds and then reemerge. Gradually extend the time you disappear to 30 seconds, and then a minute or more.
Second, if you rarely separate from baby, start making it part of your weekly routine. This will be helpful even if it's only for an hour or so. Pick a consistent time each week and make sure your child sees you getting ready to leave your home. He or she may protest at first, but resist the temptation to sneak out. In fact, involve him or her in the process of leaving. As you put on your coat, be upbeat and say something like, "Mommy is leaving but she'll be back soon!"
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