A genetic advantage allows some women to give birth over the age of 45, say researchers.
According to Israeli researchers, the reason some 45-year-olds easily have a baby while much younger women have difficulty conceiving could all be in their genes.
Although naturally conceiving a child past the age of 45 is rare, a genetic advantage appears to allow some women to do so, and Dr Neri Laufer of the Hadassah University Hospital in Jerusalem has discovered that some of those older mothers have a distinct genetic profile.
At meeting of the European Society of Human Reproduction and Embryology in Copenhagen this week, Laufer said his team had found a select group of genes that were significantly different.
Using gene chip technology, he and his team compared the genetic profiles of eight women chosen from 250 who had had children past the age of 45 with profiles of six others who had finished their families by the age of 30.
According to Laufer 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.
Conceiving naturally past the age of 45 is rare because a woman's supply of eggs diminishes as she ages and approaches the menopause, which normally occurs around the age of 50.
The women in the study were Ashkenazi Jews, who are descended from the Jewish communities of central and eastern Europe, most had had six or more children, did not use contraception and had a low miscarriage rate.
Laufer says that they challenged their reproductive system until the menopause, he adds that the distinct genetic fingerprint is not unique to this group, as he has found a similar profile in Bedouin women who also have children late in life.
He says this group of super-fertile women are unique and may serve as a model to establish what makes them so successful in terms of fertility.
But the researchers are unclear whether the late age of childbearing is linked to a delayed menopause or increased longevity, but aim to find the answer to both questions in further studies.
The research may also help scientists develop a prognostic test to determine which women are likely to conceive in their 40s.
Despite decreasing fertility and difficulty in becoming pregnant at an older age, more women are putting off having children.
Professor Michael de Swiet, of London's Queen Charlotte's Hospital, said women are getting pregnant older.