Although the hardest time for you to leave your child is probably in infancy, babies younger than 6 months old can often do fine without you for a night or two (especially if you're not nursing). That's because they haven't yet grasped the concept of object permanence -- that you exist even when you're not with them. But by 7 or 8 months, children have become aware that when you leave, you're somewhere out there, says Martha Farrell Erickson, PhD, so they're much more prone to separation anxiety. That anxiety can last well past your child's first birthday, so if your baby has a bad case, you may want to avoid traveling for a while. You also shouldn't leave town if your child has just been through a traumatic change, such as weaning or a family move.