As I pushed the stroller along a tree-lined Manhattan street when my daughter was about 3 months old, I felt elated. Sure, I hadn't slept in eons, but there was a bounce in my step that came from being out in the world. It was gloriously sunny; the sky was a bright, friendly blue; my precious bundle was drifting into a contented snooze.

Then disaster struck.

Not in my physical surroundings. But entirely, bizarrely inside my head. An image of the baby being snatched violently from the stroller by an assailant invaded my thoughts. I could see it all play out as clearly as if it were actually happening: her helpless cry as she was yanked away, me, frantic, propelling myself toward the kidnapper, screaming bloody murder as I give chase, fists ready to unleash untold harm.

In a matter of seconds, this disturbing scene shrunk and vanished, like an image on a TV screen that's been switched off, and I stood, shaken, rooted to the pavement, gripping the handle of the stroller while I steadied my breathing.

What the hell was that about? Glancing around, everything was as it should be. The sun was still out. Kylie was blissfully asleep right in front of me, pink cap cradling her delicate little head.

What could possibly have sent me from a happy-go-lucky mindset to an I-could-kill-someone rage with no provocation whatsoever? Was I, dare I ask, losing my mind?

Not according to neuropsychiatrist Louann Brizendine, author of "The Female Brain." She described what I experienced as a "daymare," a common occurrence for new moms in those first few months of parenthood. A side effect of the brain's transformation from non-mom to full-on mom.

"All of these hormones, that post-delivery, post-birth, postpartum state has got your circuitry, your 'mommy brain' triggered to be on high alert," she said. "Aggressive high alert. It's an intense surge of anger and protective hormones and fantasies."

Fantasies so dark I could be useful as an adviser to the show "CSI: NY"?

"It's actually a very normal state," Brizendine assured me. "But no one talks about it."

You can say that again. In my birthing class when I was pregnant, all sorts of helpful information was shared regarding water breaking, epidurals, breast-feeding and baby's first poop, but there was no mention of, oh yeah, those moments when you're prone to imagine despicable things happening to your baby.

Nor did any of the moms -- many of them strangers -- who came up and put a hand on my bump, offering their unsolicited horror stories from the delivery room, mention the horrors I might experience after I left the hospital.

"Moms feel like if they tell people about it somebody will think they're really crazy," Brizendine said. "You would think moms would talk about this in the mommy groups, but they actually don't. Because in the repeating of it, it almost re-traumatizes you and you're just hoping it will go away."

Brizendine said these daymares afflict about half of new moms and typically last three to six months post-birth.

And we're not necessarily talking about postpartum depression here. Occasional daymares and nightmares aside, I didn't feel depressed at all after having my child. I was tickled pink most of the time, laughing at new discoveries, marveling at this wonderful creature, relishing the cuddles and closeness of breast-feeding. But in sharp contrast to the euphoric highs, would come these random, disorienting and incredibly dark thoughts, sometimes two or three times a day.

As I was stepping into the shower, I might think, "What if someone's strangling the baby right now?" Or on the subway, I might eye up a passenger. "Does that women have a knife? Is she going to kill my husband or my baby?"

A 2007 study published in the journal Sleep showed that about 75% of postpartum women experienced terrifying nightmares involving their infants. And if you scour mom testimonials in forums online, you'll come across torment: "I'd feel a presence in the room and something (someone) would either be chasing me or strangling me" and "(I'm having) nightmares about losing my son."

My friend Heidi mentioned she'd had a dream in which her baby boy was deteriorating before her eyes but she was powerless to do anything. I'd had the exact same dream about my daughter when she was 2 weeks old. The gruesome details were uncannily similar.

Brizendine, who is director of the Women's Mood and Hormone Clinic at the University of California, San Francisco, said one of her patients admitted she had disturbing daytime fantasies about attacking her own baby with a pair of scissors or a knife.

While it might sound like this poor mom has what it takes to be the next Andrea Yates, it's important to understand that she's really just adjusting to the "fight or flight" stress hormones that for any new mom or mom-to-be are innate and incredibly powerful, Brizendine said. It may explain why actress Evan Rachel Wood recently lashed out after the paparazzi snapped a photo of her ultrasound. It explains why we say never come between a mama bear and her cub. It explains why a mother feels like she could lift a bus off her child with her bare hands if she had to.

And like it or not, Mama has to feel like that.

When a horse gives birth, its foal gets up and starts walking within 30 minutes or so. Mother sharks abandon pups almost right after birth because they're able to fend for themselves. But as any new parent will tell you, a human baby is born in an incredibly immature state, requiring its parents, particularly its mother, to be stunningly protective. That baby can't do a thing without her. Except maybe cry with Pavarotti-like lung capacity.

Which brings us to an underlying cause of these daymares and sinister thoughts -- sleep deprivation. New moms tend not to get nearly as much sleep as their minds and bodies require, and any sleep expert or doctor will tell you that prolonged sleep debt leads to hallucinations, a loss of perspective, moodiness and lo and behold, intrusive psychotic thoughts.

"Get some sleep," says psychologist Eileen Kennedy-Moore. "Sleep deprivation is one of the main causes of postpartum symptoms. Recruit your partner, a family member or a friend to watch the baby if necessary, so you can rest."

Brizendine said sometimes an intense two-week sleep program is necessary to help mom "reset the brain circuitry."