What's happening to her:
You may have noticed that your partner is acting differently following the birth of your baby (or a miscarriage). Perhaps she's crying frequently for no reason you can determine. Maybe she's showing a big change in her sleep cycles or her eating habits. Your partner might be more easily upset, or have a "flatness" of emotions. These are just some of the possible signs of a postpartum mood disorder (PPMD).
PPD, or postpartum depression is a term commonly used, but some women find that their symptoms are more in keeping with an anxiety disorder, an obsessive and/or compulsive disorder, or even exhibit psychotic episodes. Only a health care professional can determine what is happening with your partner.

What's happening to you:
You may have assumed that having a baby wouldn't change your life very much, or that everything would come naturally. You may be worried, even scared, about your partner's new emotions and expressions. This certainly wasn't what you expected (nor what she expected). You may feel isolated because you don't have anyone you can confide in. You may also feel attacked because she "takes it out" on you when things are hard for her. In short, you might think your world has turned upside down and you don't know how to make things right.

How to help her:
The most important things to do are to remind her that you love and support her and that you'll get through this together, take as much of the load as you can to eliminate even a little of her stress (this could include doing more housework or taking night-time feedings, for example), and get her to confide in her doctor. Warning: some doctors haven't a clue about PPMDs, so you may have to act as an advocate for your partner's health. Sometimes this means finding a new doctor, and sometimes it just means keeping her doctor informed about what's going on. Baby blues affect most women following the birth of their babies, and fades over the course of a couple of weeks. PPMDs are more severe and longer lasting. Treatment with talk therapy and medication are often the recipe for success. Educate yourself about PPMDs, so you can educate her doctors if necessary.

How to help yourself:
You may find that talk therapy for yourself is helpful. It's hard to live with someone dealing with a mood disorder. It's stressful for everyone involved. This doesn't mean you are weak, it just means you love your family enough to make your own mental health a priority.

Some things to remember:
PPMDs are no one's fault. Your wife isn't choosing to feel this way, so telling her to "suck it up" or "put on a happy face" will only make her feel unheard and unsupported. You don't have to understand what she's feeling, but try to understand that it's hard for her.
Some relationships undergo harsh testing in the postpartum period. Make time to connect with your wife, listen to her and try to help her in any way she asks. Your future relationship is worth the work.
She is still the woman you fell in love with. Sometimes a mood disorder can seem to erase who a person is on the inside. Remember that with the appropriate treatment, this can be a very temporary situation.
When appropriate, visit the doctor with her, or call the doctor to "catch up" on things. It's important to take an active role in her healing. If necessary, help her remember to take her meds, help prepare healthy meals and snacks, encourage her to drink lots of water, and when she is up to it, encourage her to have some "me time" so she can relax and be a person for a while (not someone's mom or wife, just a woman).
Take things one day at a time. If you are married, recall those wedding vows. This is still the woman you love, the one you took for better or for worse. Know that this CAN get better. Sometimes it just takes a little patience.

The Postpartum Husband by Karen Kleiman
This book is invaluable to anyone that loves a woman with a postpartum mood disorder. Written in an easy-to-digest format, The Postpartum Husband offers information on the illness, treatment options, tools to help her, and much more.

Postpartum Dads
Helping Men Help Their Wives Recover From Postpartum Depression. A website by and for fathers, supported by Postpartum Support International, based in the US.

Fathers Page at Radiant Mother website
Information for fathers on parenting, breastfeeding, postpartum depression, preemie babies, single fathering, gay fathers, depression in fathers etc.

When your partner has postnatal depression
An informative and useful article.