If we didn’t forget pain, women would never go through childbirth twice – or so says the myth. But the truth is that agonising memories don’t always diminish.

If you visit someone with a newborn baby, their description of the pain of childbirth often includes the phrase “never again”.  One friend even begged me to dissuade her if she ever suggested having a second child in the future. Yet a few years later there she was, approaching the due date for the next baby.  

The explanation usually given is that women simply forget exactly what the pain was like because the brain edits out the memory. In evolutionary terms it might make sense to forget the pain of childbirth in case it deterred people from reproduction. But unfortunately it seems not to be that simple.  

On the subject of childbirth, for instance, a review of studies conducted before 2000, concluded that women do not completely forget labour pain – or the intensity of the pain they experienced. It’s a difficult topic to study, though. There are scales that allow people to rate their experience of pain, but since childbirth tends to take many hours, it’s not always clear which part of labour or childbirth study participants focus on. Even individual contractions vary in their painfulness, so scoring the pain of the whole experience is never going to be easy.

What you can do, though, is examine whether individual women’s recollections of their pain change with time. A Swedish study took more than 2,000 mothers and compared ratings of their memories of the pain and experience of childbirth two months after the birth with their ratings 12 months later.

Again, the evidence suggests that most women don’t forget: the memory of the experience of childbirth was the same for 60% of the women at two months and 12 months. In fact, just one-third of the mothers at 12 months had forgotten quite how painful they’d considered childbirth at two months. And for 18% the opposite happened – far from forgetting the pain, in their minds the experience had become more painful by 12 months.

Rewriting the past

Five years later, the researchers talked to the women again. By now, the memory of labour pain had indeed declined in some women – about half of them rated the pain as less severe than they had done at two months – but for those who had rated the experience most negatively after two months, the memory of the pain simply didn’t go away. This isn’t necessarily a bad thing, though. Some studies have found that women who remember the pain of childbirth see coping with the experience as an achievement that stays with them. “If I can cope with labour pain I can cope with anything.”

But it still might come as a surprise that the memory of pain often remains unchanged with time. That’s because decades of psychological research on memory show that we reconstruct memories slightly differently every time we recall them. Our memories are not like a DVD library which we can browse through, replaying any past event exactly as it happened – instead, the context of the original memory and the way we tell the story each time makes a difference. The pain of childbirth is pain with a purpose – provided the baby is delivered safely, this positive result should, in theory, lessen the memory of the pain. But clearly, for some mothers even the happy event of a new-born baby can’t diminish the painful memory of childbirth.

So if the intensity of this kind of “positive” pain isn’t necessarily forgotten, how about more negative experiences?

Sometimes it’s important that we feel some pain and remember what caused it – the memory can keep us safe and prevent an accident from happening again. A painful cut gained when opening a tin can, for instance, will generally ensure that you are more careful the next time you’re preparing dinner. Or if, like me, you find yourself wondering whether the metal ring around a fire pit gets really hot and you touch it with all four fingers, you probably won’t do that again. Sadly that was aged 30, not aged five. As you might expect, then, people with disorders which prevent them from feeling pain at all frequently burn and injure themselves. So we don’t necessarily want to forget all pain.

Forgetting some pain might be useful, though – particularly the chronic pain sometimes associated with conditions like diabetes. Clues to controlling and eliminating this sort of pain at the molecular level come from recent work.

Just walking past the doorway where you stubbed your toe, or using the door you once shut your finger in, can make you wince at the memory. In 2006, researchers at the University of Arizona College of Medicine claimed this is because of a molecule called PKMzeta, which they argued strengthens the connections between neurons in the brain when we learn something new – and also creates physical sensitivity after a painful experience. Using mice, they demonstrated that, if they blocked this specific molecule in the spine, they could erase the extra sensitivity to pain in the mice. However, other researchers have since questioned this finding, showing that mice born without PKMzeta could still feel pain.

What about the uncomfortable or traumatic pain that can be associated with some medical procedures? When colonoscopies are done under conscious sedation, patients are sometimes given a drug called midazolam which makes them feel less anxious and also promotes anterograde amnesia – the inability to form new memories. Patients might wince or wriggle with discomfort during the procedure, but the hope is that they won’t remember any pain or feel disturbed by the procedure afterwards.

Lingering feelings

Some have questioned the ethics of this approach – and its implications. In a paper published last year Andrew Davidson, an anaesthetist at the Royal Children’s Hospital in Victoria, Australia, warned that although drugs such as midazolam can make it harder to form conscious memories, they can leave implicit memories intact. These are memories that affect us even though we can’t consciously recall them. So we can button a shirt or find our way along a familiar route to the station without thinking about when we actually made those memories. Troublingly, Davidson suggests that even if you don't remember a colonoscopy, you might feel “strangely uncomfortable when walking past a garden hose”. Would it be more ethical if patients were warned in advance that this might happen?

It’s even been suggested that to mitigate the psychological trauma of the very rare situations where patients wake up while under a general anaesthetic, they could be given midazolam as soon as the anaesthetist realises, to disrupt the formation of memories of the event. But this would only erase memories from the point when the drug was administered, not from the point of waking.

The ethics of the use of drugs like this will continue to be discussed. The very fact that they are being considered underlines the fact that – unfortunately – for people who have given birth or experienced other kinds of agony, it can be a myth that pain has no memory.

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Update: This article has been amended to reflect the evidence questioning the importance of the molecule PKMzeta.  

Even if you don't remember a colonoscopy, you might feel strangely uncomfortable when walking past a garden hose