Whenever Sarah sees a Facebook post of a smiling friend holding a newborn baby, she breaks down.

It’s not that she dislikes her friends. It’s also not that she envies their motherhood; Sarah has her own healthy baby girl. But she doesn’t have her own postcard-perfect photo like that, capturing the first moments of a new family’s joy. What she has instead are memories that continue to haunt her.

Sarah, a 28-year-old mother from Nebraska, suffers from postpartum post-traumatic stress disorder. Many people, including doctors, confuse postpartum PTSD with postpartum depression, even though the two disorders are quite different: Mothers with postpartum depression generally don’t suffer from the intrusive memories and flashbacks that plague PTSD sufferers. Instead, they most commonly deal with things like sadness, trouble concentrating, difficulty finding joy in activities they once enjoyed, and difficulty bonding with their infants. Postpartum depression is also unique to new mothers, but any traumatic experience can bring on PTSD.

In 2014, Sarah had a traumatic birth, in which her firstborn daughter was born prematurely and had to go to the neonatal intensive-care unit (NICU). The experience took a psychological toll: For months after the birth, Sarah would lie awake at night, suffering through flashbacks of the experience. Sometimes, she’d burst out crying in public.

Her condition is more similar to the PTSD experienced by veterans than it is to postpartum depression. Postpartum PTSD sufferers like Sarah experience typical PTSD symptoms like hyper-vigilance, intrusive memories, flashbacks, severe emotional distress, irritability, trouble sleeping, and nightmares, explains Anastasia Pollock, a therapist who specializes in treating trauma. Mothers who suffer from PTSD often end up structuring their lives around their disorder, doing everything they can to avoid triggers that remind them of their trauma.

The traumatic experience can come before or during pregnancy, like a bad reaction to fertility treatment or severe morning sickness, explains Jennifer Zimmerman, the cofounder of Solace for Mothers, an organization that helps women to emotionally heal after traumatic childbirths.

Other times, it can come during labor and delivery: When a newborn has a medical problem, as Sarah’s did, or when a woman feels coerced into undergoing invasive procedures during the birthing process. An experience that may not be a big deal to some women may be traumatic for others, Zimmerman explains:

“What looks like a normal labor and delivery to the outsider can be experienced as traumatic to her.”

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The psychology behind developing PTSD is complicated, but it frequently has a lot to do with expectations. Mothers often don’t anticipate having a problematic birth, so when it happens, it can leave lasting psychological scars.

That certainly seemed to be the case with Sarah.

She had a relatively easy pregnancy—but in her 33rd week, while waiting in the car for her husband to bring her ice cream, her water broke unexpectedly. Her husband rushed her to the hospital, where a whirlwind of doctors and nurses ran tests in the birthing room. A doctor said her baby had underdeveloped lungs.

When it came time for Sarah to deliver, the doctors wheeled her out of the birthing room and into a room on the NICU floor so that her newborn could be treated immediately.

Once she gave birth, the doctors whisked the baby girl away before Sarah could hold her, and her husband rushed off with them.

At that moment, Sarah told me, she remembered a couple in her birthing class that had planned the trendiest birth possible, complete with hypnotherapy and a birthing pool. At the time, she had laughed at the idea; now she envied them.

“I didn’t care about low lighting, I didn’t care about Enya playing in the background,” remembers Sarah. “I just wanted to see my daughter when she was born, and I wanted to see my husband’s face. And I didn’t get either."

The doctor cleaned Sarah up, brought her back to her room, and left her alone.

“It was grief and fear when it should have been joy and excitement,” she says. “And it was emptiness.”

A couple of hours later, nurses finally wheeled Sarah back to the NICU to see her daughter. At the sight of her baby—wires everywhere, a tube in her mouth—she felt a wave of intense melancholy. She started stroking her daughter, Eloise, but a nurse told her she was doing it wrong, she recalls—the touch was too rough for newborn skin. I can’t even touch my baby right, she thought.

A few days later, after Sarah had been discharged from the hospital, she was finally allowed to hold her daughter for the first time, though she couldn’t yet take her home from the NICU. From then on, Sarah says, she came in every day to hold her daughter for hours, sometimes with her husband but often alone. She lived off the bagels and oatmeal the hospital provided. Nurses eventually told to go home and take care of herself, a task that would turn out to be more difficult than she realized.

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When a person goes through trauma, the mind goes into an unusually hyper-aroused, fight-or-flight state, explains Sharon Dekel, an instructor at Harvard Medical School who is currently researching postpartum PTSD. The brain’s amygdala, which normally helps store memories, becomes hyper-responsive, leading to an exaggerated sense of fear. Parts of the brain that normally inhibit the amygdala stop working. When something reminds a sufferer of her traumatic experience, her unusually terrifying memories feel like more than memories; they feel like they’re still happening.

Four months after Eliose’s birth, Sarah’s niece was born prematurely and ended up in the NICU. Visiting her at the hospital, she saw the baby hooked up to all the same machines her daughter had been. When Sarah was out of sight of her brother and sister-in-law, she collapsed into her husband, sobbing.

“It all came back,” Sarah says. That’s when she realized something wasn’t right. Sarah began experience anxiety, flashbacks, and other PTSD symptoms, even bursting out crying in her church group.

“I have never gotten over being terrified that my daughter will stop breathing,” she said.

The fear is common among women suffering from postpartum PTSD. After her son was born prematurely and rushed to the NICU in 2011, Alicia, a first-time mother, grew “paranoid to pieces,” she says. Thanks to his precarious birth, she was constantly stuck in a mental moment where she felt like her son was in danger. For 14 months after he was born, she was too afraid of outdoor hazards to bring her son out of her house. When she finally mustered up the courage to take him with her to Walmart, she had a panic attack when a shopper sneezed.

She had to get rid of her coffee maker because it sounded too much like one of the hospital machines.

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Having a particularly complicated birth, such as one when the baby ends up in the NICU, makes postpartum PTSD more likely, but even mothers who have troublesome childbirths usually don’t develop the disorder.

Dekel says personal history may be one reason why some women are more susceptible than others. A woman who has experienced trauma in the past, such as sexual abuse, is much more likely to get postpartum PTSD. She also believes that neurohormonal factors could be involved, though research is still fuzzy.

No one knows for sure how common the condition actually is: According to Dekel, an estimated 1-3 percent of new mothers suffer from full-blown postpartum PTSD, but around 25 percent have one or more symptoms of it. Other researchers estimate that postpartum PTSD may affect up to 17 percent of new mothers.

While there’s been plenty of research on PTSD in veterans, literature on postpartum PTSD is sparse; doctors only began researching the disorder around 2006.

It’s hard to say why postpartum PTSD has gone unstudied for so long, though both Dekel and Pollock suspect that stigma may have something to do with it: Mothers have kept their symptoms to themselves, and so the condition has remained largely invisible. Pollock has one client in her 70s who has been carrying around her PTSD since she had her baby over 40 years ago. She only started getting treatment recently.

And because doctors only started recognizing postpartum PTSD recently, many women who have the condition aren’t diagnosed. Doctors can be quick to label suffering mothers with the better-known postpartum depression instead.

The good news: Research on the topic has been gaining speed in the last five years. Dekel is currently following a large number of women through pregnancy and beyond, hoping to identify factors that could lead to postpartum PTSD. Eventually, she hopes that mothers will be able to know their risk before giving birth.

“When we know more, we can offer better treatment,” Dekel explains. She stresses that treatment is not just important for mothers. It matters for their children, too—when your own child is an emotional trigger, bonding can be all but impossible.

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Postpartum PTSD is generally treated like any other type of PTSD, with therapy, peer support groups, and medication. Zimmerman points out that many women she works with also seek alternative therapies, such as healing through writing, art, music, or dance.

“Each woman’s path to healing is unique, and there is no one thing that works for everyone,” says Zimmerman. “It can be a process of trial and error to find what may relieve one’s symptoms, and often it just takes time to heal."

Pollock prefers to treat her PTSD patients with EMDR, a new treatment that has shown promising preliminary results. In this treatment, patients think about the source of their trauma while Pollock does things like tap on their knees and play beats, which helps patients’ brains remember that they’re in the present, not the world of their flashbacks. This helps their brains come “back online,” she says, to reinterpret their memories and put them where they belong: in the past.

“It helps them think “I’m here and now, not there and then,” explains Pollock.

Pollock treats both women with postpartum PTSD and veterans with PTSD, and she uses EMDR for both. But she also helps new mothers bond with their infants and find a new normal. While soldiers can leave combat behind, new mothers can’t just avoid triggers by avoiding their children.

Since getting diagnosed and starting therapy, Sarah says, she’s been slowly improving. Sleeping is still tough, and she still gets regular flashbacks, but she’s better equipped to react to them—they consume her mind for a few seconds, rather than a few hours.

Recently, she joined a Facebook group for mothers whose babies had been in the NICU. It made her feel better to know there are other women out there who had the same experience.

“I just think about how hard it’s been for me,” she says, “and I’m one of the lucky ones.”