Many people say that the hormonal changes that occur just after you've been pregnant can cause postnatal depression, but Dr Wheatley says that's a myth.
"Often it's the result of working women who are used to having social contact suddenly being at home with a baby, feeling out of their depth and not getting enough sleep," she says.
Circumstances do play a role, but it's not just working women who experience postnatal depression. The physical and emotional stress of having a newborn baby as well as worries about money and your relationship can trigger this depression.
Finally, if you've had depression before, you are slightly more likely to get postnatal depression.