Postpartum Depression can start anytime in the first year after delivery of a baby. In the first 1-2 weeks after birth 80% of women experience some "baby blues" which are wide mood swings brought on by the hormonal changes taking place in a woman's body during those weeks (see about.com:pregnancy and childbirth).
If the feelings of depression continue beyond that, it is then considered Postpartum Depression (PPD). Sometimes the fatigue and depression can be from anemia or low thyroid. It is best to have those things checked out in a blood test by a physician. If those items check out fine, often the depression is linked to continuing hormone imbalances (Dalton, Katharina: Depression after Childbirth. (Oxford University Press, 1985) . Dr. Dalton explains in her book that the pituitary gland doesn't send the message to the endocrine system to produce the necessary hormones, including estrogen, progesterone, and thyroid. According to Dr. Dalton, natural progesterone is safe for use during breast feeding and can help with most symptoms of PPD. With my fifth child, my doctor had me have (I believe the dosage was 100 mg) progesterone shots for 10 days, and then wrote me a prescription for 2 100 mg progesterone suppositories a day, one in the morning and one in the evening.
I felt better than I ever had after any of my other childbirths. I took 50-100 mg. of B6 and a good multi-vitamin, and made sure that I got plenty of rest. (One way to help insure that you get plenty of rest is to read On Becoming Babywise by Robert Bucknam and use his methods. I wish I'd known them with my first four!) At about 4 months postpartum, I walked a minimum of 30 minutes 4-5 days a week. I should have started at six weeks postpartum, but I still felt great even though I wasn't exercising.
I would say that my PPD was moderate with 3 of my first four children and moderate to severe after having Tessa and a traumatic delivery including severely hemorrhaging and being anemic. After having Tessa (who is #3 for me) I was almost completely cured when I started walking 4 days a week on a treadmill when she was 3 months old. I didn't even know what PPD was at the time. I just knew that I felt incredibly better after exercising for only a week.
The link above from About.com is a very helpful website. I have been searching for websites that offer natural remedies for PPD, but haven't seen any that recommend progesterone therapy so far. I know that it made my postpartum experience 95% better than the others had been previously.
If a woman has a history of bipolar disorder or prior depression before childbirth, she is much more likely to have PPD and should talk to her doctor/midwife about prevention of PPD before she delivers her baby.
The nice thing about this treatment for PPD is that it treated the root of my PPD and didn't have the side effects that other treatments can have. The progesterone did increase my appetite, but that is the only side effect that I noticed. (Another reason that I should have started exercising sooner!)
A good book to read about PPD is Behind the Smile: My Journey Out of Postpartum Depression by Marie Osmond. I like it because it has a quiz in the back the take to see if a person is experiencing PPD and helpful natural remedies that Marie used which were given to her by her doctor. I have a signed copy and got to visit with Marie at her book signing in the Phoenix area. I was thrilled that she wrote the book because I'm sure it has been read more because of her celebrity status.