Couples should not rush to fertility clinics and doctors should not intervene too fast with treatments unless there are obvious reasons for the couple not conceiving, said the study's leader, Dr. David Dunson.
Infertility is defined as failure to get pregnant after a year of trying.
Many fertility clinics report couples coming in at younger ages, but the new research shows that even couples in their late 30s have a 91 percent chance of getting pregnant naturally within two years.
The study, which analyzed details of 782 couples from seven European cities, was presented Wednesday at a meeting of the European Society of Human Reproduction and Embyrology.
Couples are waiting longer these days before starting families and experts have recently discovered that the age of the man matters too.
Research earlier this year suggested that fertility declines earlier than previously thought — in the late 20s for women and late 30s for men.
That combination has made couples more worried about delaying conception for fear of infertility, experts say.
In the United States and Britain, couples are often given treatment soon after being diagnosed as infertile and for older couples, many doctors would like to recommend techniques such as test tube fertilization earlier than one year of failure, said Dunson, a researcher at the U.S. National Institute of Environmental Health Sciences.
Dr. Basil Tarlatzis, obstetrics and gynecology professor at Aristotle University in Thessaloniki, Greece, said part of the problem is that even though doctors advise couples to keep trying for a while longer when they find no medical reason for infertility, many couples are so anxious about running out of time that they demand treatment.
Tarlatzis, who was not connected with the research, said the definition of infertility should not be so rigid and advised that healthy couples under 30 shouldn't go to a fertility clinic until they've been trying for at least two years.
Couples over 30 probably should not wait, he said.
Nor should couples of any age who have reason to be suspicious — such as when a woman has wildly irregular menstrual periods, or when either partner has had a sexually transmitted disease.
Fertility treatments, which can involve just drugs, or in-vitro fertilization — where the sperm and egg are joined in a laboratory and the resulting embryo implanted in the womb — carry risks.
Treatment increases the chance of having twins, pregnancy complications, delivering babies that are underweight or suffering from major birth defects and long-term disabilities. The older the woman is, the less likely the treatment is to succeed and the more likely are side effects.
The study found that regardless of age, the majority of healthy couples with no clear explanation for their failure to conceive achieve pregnancy by the end of the second year of trying.
It found that only 9 percent of women aged between 35 and 39 failed to get pregnant after two years, provided their partner was under 40. When the men were older than 40, 16 percent of the women couldn't get pregnant after two years.
"There is a large amount of normal variability in fertility and many couples who have below-average, but normal, fertility may fail to conceive within a year," Dunson said. "It may be appropriate to delay assisted reproduction until the couple has failed to conceive naturally in 18 to 24 months."