Women who do not require pain relief during childbirth might have a rare genetic variant, according to scientists.
Researchers at the University of Cambridge found that the variant plays a role in reducing the ability of nerve cells to send pain signals to the brain.
Around one in 100 women are thought to carry the gene KCNG4, which acts "like a natural epidural" to reduce the amount of pain experienced during childbirth.
Scientists believe their findings could help them understand more about "how we experience pain and develop new drugs to treat it".
The researchers studied a group of women who had given birth to their first child without requesting pain relief.
Tests showed that compared to a control group, the women had a higher tolerance for heat, cold, and mechanical pressure before they experienced any discomfort.
The women went through a number of exercises to test their resistance to pain, including having heat and pressure applied to their arms and asking them to plunge their hands into icy water.
Dr. Michael Lee, from the university's division of anesthesia and joint lead author on the study, said: "It is unusual for women to not request gas and air, or epidural for pain relief during labor, particularly when delivering for the first time.
"When we tested these women, it was clear their pain threshold was generally much higher than it was for other women."
Dr. Ewan St John Smith from the university's department of pharmacology, and senior co-author on the study, said: "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 labor - to switch it on.
"This makes it less likely that pain signals can reach the brain."
The team sequenced the genetic code of both groups of women and found that those being studied had the rare KCNG4 gene variant.
KCNG4 helps to produce a protein that acts like a "gate" to control the electric signal that flows along nerve cells to the brain.
Professor David Menon, senior co-author on the study, said: "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."