Why Some Women Don't Need Pain Relief During Childbirth

Some women may experience what doctors are calling a "natural epidural". Here's why.

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Childbirth is a painful experience. However, each woman’s level of discomfort and pain during labour varies substantially – to the relief of some and the loud screams of others.

Turns out there could be a reason for that. Women who don’t need pain relief during childbirth may be carriers of a key genetic variant that acts a “natural epidural”, a new study suggests.

In the journal Cell Reports, scientists at the University of Cambridge explain how this genetic variant limits the ability of nerve cells to send pain signals to the brain.

A collaboration between scientists based at Addenbrooke’s Hospital, part of Cambridge University Hospitals NHS Foundation Trust, and the University of Cambridge investigated why some mothers report less pain during labour.

A group of women were recruited by the team led by Dr Michael Lee from the University’s Division of Anaesthesia, all of whom had carried their first-born to full term and gave birth in an uncomplicated vaginal delivery, without requesting pain relief.

Dr Lee and colleagues carried out a number of tests on the women, including applying heat and pressure to their arms and getting them to plunge their hands into icy water. And the results were telling.

Image taken by Mayte Torres via Getty Images

Compared to a control group of women who experienced similar births – but were given pain relief – the test group showed higher pain thresholds for heat, cold and mechanical pressure – consistent with them not requesting pain relief during childbirth. The researchers found no differences in the emotional and cognitive abilities of either group, suggesting instead a difference in their ability to detect pain.

“It is unusual for women to not request gas and air, or epidural for pain relief during labour, particularly when delivering for the first time,” said Dr Lee, joint first author. “When we tested these women, it was clear their pain threshold was generally much higher than it was for other women.”

Next, senior co-author, Professor Geoff Woods, and his colleagues at the Cambridge Institute for Medical Research looked at the genetic code of both groups of women. They found those in the test group had a higher-than-expected prevalence of a rare variant of the gene KCNG4. It’s estimated that approximately one in 100 women carry this variant.

KCNG4 provides the code for the production of a protein that forms part of a “gate” – controlling the electric signal that flows along nerve cells. As Dr Van Lu showed, the rare genetic variant reduced the sensitivity of this “gatekeeper” – which limited the ability of nerve cells to send pain signals to the brain.

This was confirmed in a study involving mice led by Dr St. John Smith, who showed that the threshold at which the ‘defective’ gates open, and hence the nerve cell switches ‘on’, is higher.

Dr Smith explained: “The genetic variant that we found in women who feel less pain during childbirth leads to a ‘defect’ in the formation of the switch on the nerve cells. In fact, this defect acts like a natural epidural. It means it takes a much greater signal – in other words, stronger contractions during labour – to switch it on. This makes it less likely that pain signals can reach the brain.”

Commenting on the study, Zeenath Uddin, head of quality and safety at the Royal College of Midwives, said: “Every woman is different, and every birth is unique; what is important is that you get the pain relief that is right for you.

“Midwives will ensure you are getting all the available information during your pregnancy when it comes to pain control and birthing techniques, so you can make the choices that are right for you.”