Yes, it is a chemical imbalance and often as in your case, there is a marked increase with each childbirth. YOu are lucky to have a doctor who works with you on it and I applaud your recognition of it and the need to stay on the meds. Should you go for another pregnancy, please work closely with your doctor (and your family members) on you treatment. It's a serious health issue for yourself, your unborn and your current children, but one that's manageable.