1) Just a word to the wise: Formula is waaaaaay better than a depressed mama. Heck, nurse 3 days for the colostrum, and then go back on.
2) Pregnancy hormones can do keeeee-razy things. I get antepardum depression (like PPD, but while pregnant). There ARE antidepressants you can take while pregnant. If you start getting suicidal, PLEASE, seek help via your OB. Similarly, a lot of chronic depression suffers AREN'T depressed when pregnant (the hormones acting as natural antidepressants). It splits (80/20?) though on PPD on top of normal depression afterwards, or just back to normal depression. Make sure someone you trust to call 911 is around for the first few weeks after birth, as you'll be at much higher risk for PPD and psychosis. Again... breastmilk isn't best if mama is dead. Or if mama has killed her baby. Or even "just" cannot bond with her baby. (Could you be fine? SURE! But really, leave the formula option on the table guilt free.)
3) There is a short list of antidepressants that are widely used by nursing mothers. I'm not up to date on the longitudinal studies on these.
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Myself... Antepardum depression runs in my family, so it didn't even occur to me to mention anything to my OB. "The Suicide Hours" I just clung to a chair and screamed and cried and shook, or stayed catatonic silent, for 1-2 hours a day for 6 months. ((Both ways, just trying not to off myself)). The rest of the day was soooo much better than those hours (like clockwork, every day starting somewhere around 5pm-7pm), that even though biserable, they were totally handleable.