• Not all women have the same level of pain during childbirth
• A study found that some women had higher pain tolerance for heat, cold and mechanical pressure
• These women had a higher-than-expected occurrence of a rare variant of the gene KCNG4

A genetic variant that acts as a natural pain reliever may explain why some women don’t require pain relief during childbirth, researchers say.

The level of pain and discomfort experienced during childbirth varies widely, so researchers at the University of Cambridge in England decided to investigate why some women have less pain during labour and delivery.

“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 co-lead author Dr Michael Lee, a consultant in pain medicine in the Division of Anaesthesia.

“When we tested these women, it was clear their pain threshold was generally much higher than it was for other women,” he said in a university news release.

A rare genetic variant

Lee and his colleagues assessed women who didn’t request any pain relief during an uncomplicated vaginal delivery of their first baby. To test their pain threshold, researchers applied heat and pressure to their arms and had them plunge their hands into icy water.

Compared to a control group of women who required pain relief when giving birth, women in the test group had higher pain tolerance for heat, cold and mechanical pressure.

No differences were found in the groups’ emotional and thinking abilities, suggesting a difference in their ability to detect pain.

Genetic tests on both groups of women found that those in the test group had a higher-than-expected occurrence of a rare variant of the gene KCNG4. About one in 100 women have this variant.

This variant limits the ability of nerve cells to send pain signals to the brain, according to authors of the study published on 21 July in the journal Cell Reports.

New drugs to manage pain

“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,” said study senior co-author Ewan St John Smith, a group leader in the Department of Pharmacology.

“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,” Smith said in the release.

Researchers said they hoped the discovery would lead to development of new drugs to manage pain.

“This approach of studying individuals who show unexpected extremes of pain experience also may find wider application in other contexts, helping us understand how we experience pain and develop new drugs to treat it,” said study senior co-author Dr David Menon, head of the Division of Anaesthesia.

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