A genetic variation may explain why some pregnant women forgo an epidural, research suggests.
While childbirth is widely recognised as painful, the level of discomfort mothers-to-be experience varies significantly.
To learn more, scientists from the University of Cambridge analysed a group of women, some of whom did not opt for pain relief during labour.
DNA sequencing revealed many of these women expressed a variation of the gene KCNG4, which is carried by around one in 100 females.
KCNG4 codes for the production of a protein that controls the electric signal that flows along nerve cells.
The genetic variant is thought to reduce the ability of these nerve cells to be “turned on”, acting as a “natural epidural”.
Mutation ‘makes it less likely pain signals reach the brain’
Many pregnant women opt for an epidural during childbirth, which numbs part of the body.
Although generally considered safe, it has been linked to low blood pressure, temporary loss of bladder control and nausea.
With some women forgoing the anaesthetic, the scientists looked at a group of women who gave birth vaginally at full term without pain relief.
These were compared against others who had similar deliveries but chose to have an epidural or other analgesic.
All the women underwent a number of tests to measure their pain threshold, including applying heat and pressure to their arms, and plunging their hands into icy water.
Perhaps unsurprisingly, results – published in the journal Cell Reports – revealed those who did not have an epidural or similar relief coped better during the tests.
“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 lead author Dr Michael Lee.
“When we tested these women, it was clear their pain threshold was generally much higher than it was for other women.”
DNA sequencing later revealed a higher-than-expected prevalence of the KCNG4 variant in the group who did not have pain relief.
A mouse study then confirmed carrying the genetic mutation means the level at which nerve cells are “switched on” is higher, possibly explaining why women who express the variant experienced less pain.
“The genetic variant we found in women who feel less pain during childbirth leads to a ‘defect’ in the formation of the switch on the nerve cells,” said Dr Ewan St John Smith, lead author of the rodent trial.
“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.”
The scientists hope their discovery will “open avenues to the 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 author Professor David Menon.