Here is an excerpt from a wonderful article I located last night regarding this topic. The entire article is absolutely fascinating and DEFINITELY worth the read.
Hennaing a woman after she gives birth is a traditional way to deter the malevolent spirits that cause disease, depression, and poor bonding with her infant. The action of applying henna to a mother after childbirth, particularly to her feet, keeps her from getting up to resume housework! A woman who has henna paste on her feet must let a friend or relative help her care for older children, tend the baby, cook and clean! This allows her to regain her strength and bond with her new baby. She is also comforted by having friends who care about her well-being, and is helped to feel pretty again. It’s a comfort to have feet beautified when you haven’t seen them for several months. The countries that have these traditions have very low rates of postpartum depression.