The rising hormone levels that gradually effected the incredible changes in your wife's body during the time she was carrying your daughter have now precipitously dropped. Usually this lasts for a few days or less and is quickly forgotten. It's not unusual, however, for the blue period to come and go for six weeks. Moms who have adopted their babies also commonly go through a blue period. We do know, though, that even at this exciting time of having a newborn, there are good reasons to be blue.
Every new beginning is also an ending of what was before.
Every ending is a beginning. Whenever a baby is born, the world will never be the same. This is wonderful. It's also okay to grieve for the loss of the way life was before. Your wife no longer has the control of her own time the way she once did.