Scientists from the University of Cambridge compared 33 women who did not request any pain relief during an uncomplicated vaginal delivery of their first child, with a control group of 33 women who were given pain relief during delivery of their firstborn.
Two women in the first group had the KCNG4 gene - a much higher proportion than average.
They carried out three tolerance tests on the women for cold, heat and mechanical pressure.
The tests involved dunking their hands in icy water, having a hot thermode attached to their forearm and contracting a cuff on their upper arm.
The first group of women, containing the KCNG4 gene carriers, were found to have a greater ability to withstand pain, which could account for why they did not request any pain relief during childbirth.
Dr Michael Lee, from the Division of Anaesthesia, said: "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.
"When we tested these women, it was clear their pain threshold was generally much higher than it was for other women."
There is, therefore, a higher degree of pain tolerance in a group of women among whom KCNG4 carriers are over-represented, the study suggested.
KCNG4 inhibits the ability of nerve cells to send pain signals to the brain, where the sensation of pain is then registered.
This means that women who carry the gene should have a higher pain threshold during childbirth, as it takes very strong contractions to "switch on" the pain process.
Dr Ewan St John Smith, the senior co-author, 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 labour - to switch it on. This makes it less likely that pain signals can reach the brain."
It is hoped that the study, published in the peer-reviewed journal Cell Reports, will aid the development of new drugs to help control labour pain.
Now scientists know the KCNG4 gene makes it harder for nerve cells to transmit pain messages, future pain relief could revolve around the development of drugs which block these pain messages in a similar way.
Professor Frank Reimann, co-author, said: "Not only have we identified a genetic variant in a new player underlying different pain sensitivities, but we hope this can open avenues to the development of new drugs to manage pain."
Scientists also believe the discovery could lead to better treatments for other types of pain, not just that during childbirth.
Professor David Menon, senior co-author, 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."
