It sounds like she needs some professional help anyways, but hopefully some information will let you know whether or not she really needs to be hospitalized and how urgent this is all needed.
I believe you said that the new baby is 3 months old, so baby blues and normal adjustment are likely not factors here.
Clinical Postpartum Depression manifests in 4 different ways:
-Loss of appetite/overeating
-Difficulty sleeping (which is a hard "symptom" to diagnose)
-Loss of interest in activities, boredom
-Difficulty caring for self or infant
-Thoughts of suicide or running away (very common, risk of action is around 5%)
-Feelings of hopelessness
-Panic attacks -- trembling, shaking, hot/cold flashes, sweating, faintness, dizziness, shortness of breath, chest pain, choking sensations
-May be combined with other manifestations, especially OCD
Obsessive Compulsive Disorder
-Intrusive, random, disturbing thoughts, mostly about harm coming to the baby
-Typical OCD symptoms (hand washing, turning lights on and off, making sure doors are locked, checking on baby obsessively, etc)
*It is a good thing to note that moms who have thoughts about harming their babies statistically almost NEVER harm their babies, and you can tell these moms this. The thoughts come from the anxiety and worry about things that can happen to the baby, and thoughts come from this FEAR of such. This usually makes mothers hypervigilant, AND this is quite different from postpartum psychosis -- which is what Andrea Yates had (many moms have the fear that they are just like her).*
Post Traumatic Stress Disorder
-Feeling disconnected with baby
-Avoid talking about the experience or talking about it over and over
-Birth experience was traumatic
-Woman has experienced sexual assault
These manifestations are NOT cause of hospitalization, but patients will benefit from treatment -- it may be helpful to remind women that they do NOT need to sacrifice for their babies. What babies need is a happy mom, even if that does need medication. Medications like Zoloft and Paxil (SSRIs) have no adverse effects on infants, and I would recommend that you have her go to a psychiatrist instead of a family doctor. Postpartum moms need to be given doses carefully and work up slowly, because their bodies are so sensitive to the chemicals.
Psychiatric Emergency -- usually requires hospitalization
-Feeling speeded up
-Little need for sleep
-Very distractible and irritable
-Speech is fast and pressured
-Easily susceptible to anger or disappointment
*Symptoms need to be lasting at least 4 days -- do NOT leave this untreated*
-Extreme confusion or distractibility
-Inability to distinguish reality from fantasy
-Irrational changes in mood
Psychosis is WAY different than thoughts about harming the baby. It's more than worries, it's beliefs that only make sense to the mother. For example, Andrea Yates thought she was the devil. To her, if she killed her children, she would get executed, and this would kill the devil, thus saving the world. To her, it made sense. See the difference between this and OCD PPD?
Mothers HAVE to be medicated for mania and psychosis, but usually the mother cannot breastfeed while on these medications. Make sure you reassure her that what she needs is to be better for the baby, and even if the baby has to have formula, it'll still be better than her being miserable. Check with friends or a milk bank for extra breastmilk, if it's been tested for AIDS and Hepatitis. Exercise with caution.
I think it's important that we not try and diagnose the mother, because we're not really qualified to do so. The best thing to do is refer her to a psychiatrist immediately, because it sounds like there are many factors at play here.
Hope this helps!