By Michelle Roberts
BBC News health reporter in Copenhagen
Scientists have identified a genetic profile that appears to enable women over 45 to conceive naturally.
Fertility usually dips past the age of 40
A team from Israel's Hadassah University Hospital believe the discovery could help improve fertility treatment for older women.
They told a major European fertility conference how eight Ashkenazi Jewish women who had babies naturally later in life all shared the special genes.
The researchers believe the genes might help slow ageing of the ovaries.
They are known to play a role in cell death and DNA repair.
It is hoped the work could help the development of better treatments for infertile older women as well as further understanding of the ageing process.
Special genetic fingerprint
The Israeli team now plans to study women from other ethnic backgrounds to see whether the same genetic pattern exists.
Lead author Dr Neri Laufer said: "We already have preliminary results demonstrating similar results from another group."
For their initial research they studied a group of 250 women aged 45 and older who conceived naturally.
They took blood from eight of these women for genetic analysis.
Compared with six women of the same age who had finished their families at the age of 30, the eight women shared a special DNA fingerprint.
Dr Laufer said: "These women appear to differ from the normal population due to a unique genetic predisposition that protects them from the DNA damage and cellular ageing that helps age the ovary.
"What we do not yet know is whether this reproductive success is linked with potential longevity," he told the 21st annual conference of the European Society of Human Reproduction and Endocrinology.
To date, the oldest woman recorded to have a baby was a 66-year-old woman from Romania.
However, she underwent fertility treatment for nine years before becoming pregnant.
Dr Laufer believes that identifying women with the genetic fingerprints that they have found would enable doctors to know which women are still fertile in advanced age and may determine the counseling they need.
"However, the question of motherhood over the age of 45 is a delicate and complex one. It is very dependent on the religious and cultural background of the women in question," he said.
There is a theory that merely having a lot of children can increase the chances of future pregnancies.
But Dr Laufer said if this applied to the Ashkenazi Jews it should be accompanied by a higher than usual miscarriage rate. In fact, their miscarriage rate was extremely low.
The number of women having children in their 30s and 40s has increased sharply over the last 20 years.
Overall, there were 621,469 live births in England and Wales in 2003 - an increase of 4.3% on the 2002 figure.
The largest increases - by more than 7% - were recorded in women aged 35-39 and 40 and over.
The average age of pregnancy is now 29.4 years, and for first time mothers it is 27.4 years, according to the Office of National Statistics.