Why can some 45-year-olds easily have a baby while much younger women have difficulty becoming pregnant?

The answer, say Israeli researchers, could lie in their genes.

Having a naturally conceived child past the age of 45 is rare. But Dr Neri Laufer of the Haddassah University Hospital in Jerusalem has discovered that some of those older mothers seem to have a distinct genetic profile.

"We found a select group of genes that were significantly different," Laufer told a medical conference Tuesday.

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.

"These women appear to differ from the normal population due to a unique genetic predisposition that protects them from the DNA damage and cellular aging that helps age the ovary," he said.

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.

CHALLENGING THEIR SYSTEM

All the super-fertile women in the study were Ashkenazi Jews, 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.

"They challenged their reproductive system until the menopause," said Laufer, who added that the distinct genetic fingerprint was not unique to them.

He found a similar profile in Bedouin women who also had children late in life.

"This unique group of super-fertile women may serve as a model to learn about what makes them so successful in terms of fertility," said Laufer.

But the researchers do not know whether the late age of childbearing is linked to a delayed menopause or increased longevity. They will try to find the answer to both questions in further studies.

"We hope that better insights into these less than 50 genes will help us in the future to develop better fertility agents or the ability to manage infertility in women over 40," Laufer added.

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.

"People are getting pregnant older," Professor Michael de Swiet, of Queen Charlotte's Hospital in London, told the European Society of Human Reproduction and Embryology meeting.

In Britain in 2000-2002, about 10 percent of women were 35 years old during their first pregnancy, compared to just 3 percent a decade earlier.

Most fertility treatments are given to women between 30-39 years old. De Swiet said the idea age for having children is between 25 and 35.