Two common treatments for fertility problems do not work, an Aberdeen University-led study suggests.
UK guidelines recommend the drug clomid and artificial insemination for couples who have trouble conceiving despite no known cause for their infertility.
But trials of 580 women in Scotland found the treatments were no better than trying to get pregnant naturally, the British Medical Journal reports.
A third of couples who struggle to get pregnant have unexplained infertility.
It means, despite a battery of tests, doctors cannot find a reason why they struggle to conceive.
Professor Siladitya Bhattacharya, study leader
The options listed by the National Institute of Clinical and Health Excellence (NICE) include up to six cycles of unstimulated intrauterine insemination - where sperm is inserted directly into the woman's womb - and use of clomid (clomifene citrate), a drug which stimulates the ovaries.
The treatments have both been offered for many years because "doing nothing" is an unpopular choice among patients, the researchers said.
But until now there has been little rigorous testing of their effectiveness.
Five hospitals were involved in the study: Aberdeen Royal Infirmary, Edinburgh Royal Infirmary, Ninewells Hospital in Dundee, Falkirk and District Royal Infirmary and Glasgow Royal Infirmary.
Scientists from Oxford University also took part in the research.
In the six-month study, participants had all suffered unexplained fertility problems for more than two years.
Overall, 101 women ended up having a successful pregnancy.
Of those who tried to conceive naturally, 17% became pregnant and gave birth to a live baby.
For clomid, the birth rate was 14%, and insemination 23% - not significantly different from the chances of success with no intervention.
However, the women undergoing active treatment were more reassured.
Between 10 and 20% of women taking the drug had side-effects, including abdominal pain, bloating, hot flushes, nausea and headaches.
The drug also increases the risk of twins and triplets, which can be more dangerous for mother and babies than a singleton pregnancy.
But the researchers were quick to point out that it is a very useful treatment for women who have problems with ovulation.
'Cheap but common'
Study leader, Professor Siladitya Bhattacharya, from the University of Aberdeen, said the findings challenge current practice.
"The first thing is it sends a positive message that the chances of success spontaneously are pretty convincing."
He added: "These treatments are cheap but common - if you total all the money, time and effort spent on them, it's considerable."
In an accompanying article, researchers from the Assisted Conception Unit at Guy's and St Thomas' NHS Foundation Trust, said because of the lack of evidence, many couples with unexplained infertility endure "expensive, potentially hazardous, and often unnecessary treatments".
Dr Allan Pacey, senior lecturer at the University of Sheffield and secretary of the British Fertility Society, said: "It is very important that we have a strong evidence base to support the treatments that are offered to infertile couples and as such this study is very valuable.
"Intrauterine insemination has been offered to couples with unexplained infertility for many years, but if there is no benefit in doing so then we should re-evaluate the clinical guidelines so that NHS money is used wisely."